Welcome to the Forum.
Please know that what you and your son are experiencing is not that unusual. As you can see from the many posts on this Forum, young children do remember previous lives, and are sometimes confused by what happened before, and their present-life experiences. You can help your son resolve his confusion by understanding that he needs to talk about his memory with someone who will listen--preferably you and your husband.
I go into great detail in Children's Past Lives why these memories come up and how to talk to a child about them. So please get a copy of my book ASAP and start reading!
In the meantime, when your son says something about his other mom, take a deep breath, calm yourself, and ask open-ended questions. Engage him in conversation, because he's trying to communicate something very important, probably something that is troubling him. By using open-ended questions, or reflecting his words back to him, you can keep the flow of the conversation going, which would be in his best interest. The worst thing to do is to shut down the conversation. Even if you don't understand what's going on with him, engage him, enter his reality, and see what he's trying to communicate to you.
Often, the past life memories surface because children are recalling traumatic incidents from previous lives. (I know that sounds bizarre, but it's true.) By allowing them to speak freely about them, they can release the emotions they're still feeling from the past.
In your son's case, does he have separation anxiety? Any behaviors, fears, or strong emotions that seem incongruous to his present life? There might be some clues in his present personality that relate back to this other life, but didn't really make sense before. If you can encourage him to keep talking as he brings up this memory, he will tell you what happened. It's all right if he gets emotional about it. Just let him talk, and then reassure him that he's now safe, or that you're his mother now, or whatever you feel he needs to hear.
Just know that there is nothing wrong with your son. Children have past life memories, and some are traumatic, and others are not. If your son is remembering a traumatic event, you can help him release the feelings connected to it.
Feel free to keep posting. We're here to support you and your son. And please read the book and the posts in this section. You will see that you are not alone, and that you can help your son.