There is another way of seeing it: The dynamics are such that your BF is in the Parent role and his mom is the Child (dependent on him for her financial and emotional needs.)
It is a codependent relationship. If you are going to resent the one, it would be fair to also resent the other. (Not that resentment will resolve anything, just that the situation doesn't justify blaming just one of them.)
I was your BF. How it plays out is that the Parent feels over-responsible, and develops ineffective coping skills (control, perfectionism, etc.) to help meet the Child's or Children's needs.
The dynamic in your relationship/marriage will
look the same but it won't have ANYTHING to do with his mom. It will be about
his need to be responsible and
your need to have someone be responsible.
Already you are needing his mom to be responsible for what you perceive to be your BF's, and, by extension, your problem.
But as the saying goes, "opposites attract" ...his over-responsibility has attracted your under-responsibility, and vice versa. You and your BF are playing out a codependent relationship dynamic, too. (I know, it is harder to see when one is involved in it -- much easier to observe when it is going on "out there".)
When you get married, you will gain a Parent and your spouse will gain a Child. It happens all too often, since most of us are engaged in codependent relationships of one or another level of severity. You say you already do feel like a Child, with the two of them -- but it is still your BF who is in the Parent role there. It's not an age-thing but an energetic or dynamic-thing.
At this point, it doesn't really matter WHY he is over- and you are under-responsible. The excellent thing is that you've recognized it, and now know that you need to work
together to pave the way for a long, happy and successful relationship together. It is your mutual problem, and his mom is just reflecting your own "stuff" back at you -- love, don't resent, her for that
You may wish to read 'Codependent No More' by Melody Beattie, and learn about yourself first. (Of course, you'll simultaneously be learning more about him, as well.) It'll help you to present what you learn in a loving, non-judgmental way, so that you can both learn to relate Adult-to-Adult. It always goes across easier when we say, "Look, this is about
us," instead of, "Hey, here's
your problem...and I need you to fix it!"
Best of luck -- it is sometimes tough work, but very important to a happy future.