I’m disturbed .. how should I go about this.

I’ve been with my boyfriend for 4 months and he had a family party and invited me so I can meet everyone. Well long story short he had a little cousin who’s 13 who’s a perv. I was waiting to use the restroom and he was in there and when he came out I was gonna go in next. Next thing I know, this little fucker grabbed both my boobs and pinched my nipples!!!! I was like WTFFF! I told my BF and he said let it go he’s just a kid... what should I do.