He does seem to be preoccupied and very close to his mother. It is admirable that he cares so much about his mother but as his girlfriend, it is understandable that you do not like being compared to her. It’s also a bit unusual. It may be that he uses his mother as a comparison because he has no other women in his life to compare you to. His mother might be his frame of reference for how he understands women.
You also mentioned that the three of you sit together on the couch and he feels compelled to move closer to his mother instead of you. This behavior also seems abnormal. It would be interesting to know how close they sit in proximity to you and how often this occurs.
What makes this question difficult to answer is that I do not have enough information to offer you an explanation as to why this might be occurring. It’s also difficult to say with certainty that their behavior is atypical for a mother and son. If I had more information I may be able to make this determination.
If his behavior is a problem for you, then it is a problem for the relationship. You said that you talked with him about this issue and he does not understand what he may be doing wrong or what it is about his behavior that bothers you. This is also a concern for the relationship. The two of you seem to be disagreeing as to whether there is a problem.
You can and should try talking with him again. When and if you talk to him about this matter, approach him in a non-blaming (i.e. you’re ruining our relationship) and non-threatening (i.e. pick me or your mother) manner. Tell him what specific behaviors are bothering you and how you would like him to change his behavior. If you talk to him in a non-confrontational way and are direct and concrete about how you want him to alter his behavior with his mother, then he may be able to or may be willing to make the changes that you believe he needs to make. Having this conversation may help save the relationship.
If you talk with him and he still does not change, then you may have a tough decision to make. You will then have to decide how much you can tolerate his behavior or whether you want to stay in the relationship. I hope this helps.