Im fifteen and I've dated my boyfriend for almost a year, we're pretty serious about each other and have made out before. I know i should do what im comfortable with but i dont know what i am comfortable with.
DONT DO ANYTHING YOUR NOT COMFORTABLE WITH!! But you guys have been together for a reasonable thing. If I were you I would let him and take it little by little don't let him get the idea that he's gonna get more or that he can touch you like that whenever he wants.
Your fifteen don't worry about that stuff go have fun. You don't need some guy grabbing you boobs you will have plenty of time making out and getting you boobs grabbed when you marry. I didn't grab my first boob till I was 19. Sexual Stuff like that isn't what a true relationship is about and if all this kid want to do is grab your boobs then he doesn't respect you as a young lady. If you don't know if you comfortable with him doing that to you it means your not and you should never do something you not comfortable with.
Don't. If you're not sure, and 100% comfortable, don't do anything sexual. You don't know what you're comfortable with, so until you figure it out, don't do it, you'll regret it later. You got so much time to get your boobs touched, that taking your time to feel comfortable won't do any harm.