I'm no psychologist, but I was raised in a family of 6 kids, have dozens of cousins and now have 3 children of my own--and your cousin's behavior is not normal for a 7 yo. He's no innocent--he stopped his behavior when his mom came in the room, right? So he knows what he's doing. Don't be fooled by the big innocent-eyed look he gives you if you push him away, he's playing you for a sucker.
Keep your distance from him. Lock your door at night, or put a doorstop under it (they're cheap and they work). If he touches you again, make it clear that he had better never EVER do it again!
In the meantime, I'd talk to another adult women in my family about what he's been doing (if not the boy's mother--at least not right away, not yet)--believe me, if they have children of their own, they'll want to know about what happened to you so they can protect their own kids.
--because that's the other problem. You have to worry about the other young women in your family or neighborhood, because a boy like him is going to take advantage of them sooner or later--especially as he grows older and larger in size. He sounds like he lacks a conscience, and I'd worry a whole lot about that.
In summary: 1. Stop the boy from ever touching you again, make it clear to him that he'd better not,--but don't at this point say "I'm going to tell your mother," because then he could get to the mother with his own story first. 2. Confide in at least one other adult member of your family about what happened, so you have someone close to the situation to talk to. 3. Keep your distance, but also keep tabs on what the boy is doing at family or community gatherings (which girl he's hanging around, how he's acting). 4. KNOW that this boy is probably going to go on to hurt a girl or young woman as he gets older--best for your family to know about it now. The more he gets away with, the more dangerous he will become.
6. I think ultimately the mother will have to be told, but right now talk with other people.