Well, having been in similar circumstance at your age, I would first do more than decide that you are going to live together.
How are you going to pay your bills?
Who is signing the lease? Both? One of you? If you (or he) decides that the living arrangement doesen't work out to either of your satisfations, what will you do? Move back in with Mummy?
Believe me, that would be the worst.
Is she supporting you in any way, shape or form? How will you support yourself if you lose your Mom or your boyfriend?
Do you have student loans? How will you pay those after graduation?
I know I sound like a kill-joy, however, I got zero advice and had to move into the worst dive because I absolutely would not move in with my JW Mom. I lived in a literal ghetto hovel with revolving roommates due to my money issues.
However, it was better than living with a rabid JW. And, I paid all my bills. Then I moved in with my boyfriend. 2 years later. When I knew I could do it on my own. That was the key.
Can you? Can you survive without anyone helping you?