Age problem

Faith
I'm 15 and my boyfriend turned 18 just yesterday and I was okay having sex...but now whenever he touches me or talks about it all I can think about is if my grandparents(who I currently live with) would find out they would call the police and he would be a registered sex offender and I can't do that to him but he just doesn't understand my worries. Does anyone have any helpful insight or ideas or words of wisdom?