Ok I just read through your thread and I had a moment where I was shaking, because I recognized a lot of your son's references. I see that you did find out how Liang is pronounced in Chinese!
After reading everything, this is my understanding of your son's past life. As you were deducing, tattooing, fishing and worshiping the water spirits occurred in the Jiangsu and Zhejiang provinces. The majority of people from those provinces are Han Chinese, which makes up one of the largest ethnic groups in China. The Han have their own dialect (which in Asia, is a separate language and not a regional way of speaking that is mutually intelligible with other English speakers, like in the US), their own style of dress and so on. Han Chinese culture is the blueprint for Korean culture, so it's no surprise to me that much of what your son has said may be Korean sounding in nature, as he may have been a native Han speaker and he had been using his own nick names for his king and soh-soh.
A majority of everything else he said points to the late Qing period, during the middle to late 1800's in China. Red and yellow are the predominant colors in the forbidden city (imperial palace) in China. Yellow is strictly for the use of the emperor and red symbolizes luck, joy happiness and is used for weddings.
The armor that you found, was also used by the imperial guards. If this is what your son identified as being the correct armor, that again points to the late Qing period, because the armor is actually Manchu armor and not Han. The Manchus are from Mongolia and are an ethnic minority in China, but since they were the last imperial dynasty in China, their influence is much of what we consider to be "ancient" or pre modern China. If your Korean friend identified that as Korean armor, he is mistaken, I am sad to say. He's not totally wrong though, much of the armor looks very similar.
Although I can't place the name anywhere, I would not be surprised that center sky hill is the name of a temple or sacred place in the forbidden city or somewhere in China. It just sounds Chinese... I just know it, even if I can't put my finger on it. It sounds like a literal translation of something and could be one of the mountains your son was talking about.
The black substance that your son said he had to eat, I'm almost certain was a ball of opium. Small balls of it were ingested as a medicine by the Chinese, but it was more commonly smoked. Opium was introduced to China in the 1700's.
The biggest and final clue that I think I know 100% about what your son is talking about and what made my heart stop is that the name of his beloved soh-soh sounds like cee-bee or sexy. I just knew who he was talking about and my heart stopped right away. He's talking about
the Empress Dowager Cixi, her name is pronounced she-zi. However, if you pronounce it like it's spelled in English, it's said six-e, which sounds like sexy. You can listen to some pronunciations
here. She became a concubine of the Qing Emperor Hsein Feng (Xianfeng) at age 16, who must be the king your son was talking about. I think you said he mentioned fang at one point and maybe he was talking about him?
This may be an off chance but the man in black your son spoke about, may have been an important eunuch within the palace. Eunuchs during the late Qing period, even though they were technically servants of the royal family, were much more like mafia bosses in reality, having gotten rich by stealing and selling many of the palace antiques and treasures. If your son was a "bad" man in his past life, he may have had dealings with one of the higher ranking eunuchs.
The second biggest confirmation that I have that it was Cixi, is because you said:
He's always said she had dogs
Cixi was famous for bringing dogs into the imperial palace. She may have had three favorites, but she had tons, literally. This is where it gets kind of crazy.. how do I know all of this? Because Cixi chose a young Puyi to become the next emperor of China and when he was 16, in my past life, I married him and lived in the imperial palace with him and took over Cixi's packs of dogs. Years ago, I actually had a woman who was seriously into the dog breeds of kings and queens and she had written an email to me and identified the many breeds in the photos along with some innocuous buildings that turned out to be kennels.
These photos are of my past life and some of Cixi's dogs:
I don't know if Cixi's favorite color was red, although it is mine. I would definitely do some research into her, she has many biographies and articles written about her as she is one of the most famous Chinese women in modern history. You may also learn something about your son or the emperor in the process!
Last but not least, my heart also skipped a beat to hear Cixi's name as I've believed for many years that she was also my grandmother in this life. She died in the early 90's and I think most souls rest around 10 years before reincarnating. If your son is 15 or 16 now, he must have been born in the early 2000's and there's a good chance that she is the little blond girl your son is talking about.
For the connections with Korea, it could quite possible that your son and his friend (who may have lived in China as a Han with him) chose to live there as Korea is very close to Han culture. Most of my extended royal family reincarnated here in the US with me (or vice versa I should say) and if you look at the changes in China over the last 80-90 years, I think it's safer to have lived outside of it if possible. I know for my family, that's what we did.
**I'm just going to throw in an edit, just for the sake of doing so, that as others may have pointed out, your son could be talking about multiple past lives.
I agree with what you've probably guessed that ornament sized doll house, was something that your son viewed from the girls life, now. They could have met in a life previous to the one he mostly talks about, where they ran away and lived in a hut. That could have been in Korea.
It's possible that his friend or even he himself came from Korea and lived in the Imperial palace with the emperor and Cixi, although that would take some documentation to support as it's just speculation.
Regarding the man in the black suit, I was also thinking that either he could be describing a eunuch or another person when they were younger, with black hair and then he was remembering when they were older, with white hair.
Also, he could be talking about a foreigner. I don't think the Chinese called their clothes a suit back then. From the 1700's onward, parts of China were like occupied Berlin during WWII and many foreign powers were competing for influence in China, even to the point that many countries anchored ships on a near permanent basis in the harbors, just to maintain a presence there. So he could be talking about a blonde haired foreigner in a black suit and maybe he meant the man also wore a matching black hat.
I'm also curious about the city of the dead because the people and terminology he uses makes me think of Scandinavian people and folk lore. He called them northies and I think he mentioned something about the north pole too and these men or man had blonde hair and a hairy face. He may have very well have met sailors from Norway, Denmark or even Canada. I know on one occasion I met the Canadian prime minster and his wife. Maybe he really just liked these men and projected them into his experience in the city of the dead.
I just find this fascinating as I'm from the same time period and there may be a connection here to my own grandmother.