I guess I'm curious to know if this is a guy you know and were friends with before he asked you to be his girlfriend. I had a boyfriend at twelve and I look back on it now and I kind of regret that I let it get so serious at such a young age. I mean, we didn't do anything really "bad", but I let the "relationship" consume my life. I lost all my friends, I lied to my parents. But it lasted for six months, which is forever and a day in pre-teen years
I think it is awesome you talked it over with your mom. I always had a real open relationship with my mom too and it's amazing. I would say regardless of what any of us advise you to do, stick with your mom's advice and what your common sense tells you is the right way to handle all of this. I don't know how boyfriends and girlfriends interact in junior high schools these days. I know back when I was in junior high, couples were not allowed to show public displays of affection (no hugs, hand holding, kissing, etc.) I'm sure a lot of kids got away with it, but if the teacher or another school administrator saw you, you got in trouble.
But there are lots of fun things you can do, like do homework together or go out with a group of friends. We used to play minature golf a lot. Haha do putt-putt courses still exist? I haven't seen one since I was in high school.
Anyway, keep level-headed, listen to your mom and just take it one day at a time. And have fun!