He wasn’t clinging to his family, he was controlling them. The “all together“ thing was something they had outgrown but Billy was forcing them to abide by it. That’s why he got mad at Freddie doing a solo save. Mary held off going to college but she was trying to leave. Billy didn’t know that was going to happen to him after turning 18 so he was trying to be Captain Marvel, even though he wasn’t great at it, to the point of smothering them.
The thing with the mom was still the attachment thing from the first movie. He thought they would abandon him at 18 so he kept it formal instead of calling her mom.
Billy is clinging to his family because they have strengths where he was weaknesses. That's where the impostor syndrome comes in. He begs Mary for help because she's smarter than him, and Pedro was the one who knew about Steve even though Eugene mapped the whole lair. He also is afraid to grow up because Mary tells him he needs to get a job if he wants to stay in the house, or else Victor and Rosa won't be able to afford to keep him.
I don't buy that Billy "kept it formal" with Rosa. He literally says he had to hang on to them because he finally found a family. I don't buy that he'd go all in like that just to be on a first name basis with the closest thing to a mother he had.
Though ultimately none of that mattered because Billy gave them all their powers back, we learn that Victor and Rosa paid off their house, and they all have dinner night like normal.
