Kanna, I had a chat with my partner about this - for many years she was the boss psychologist of an organisation which supports people with CP.

Her general comment was that what you have described about the relationship between your BF and his mother is not uncommon.

She went on to say that, assuming that his physical health issues are being monitored, she knows of many adults with CP who have happy relationships with people who don't have CP, though she did suggest that you and your BF work out how to set firm relationship boundaries between him and his mother (this may not be easy).

I wish you and your BF every happiness.