I'm being blackmailed for nudes and sex

So for about 6 weeks or so , I've been having  sex with my neighbor, he is 18, and I'm 17. Obviously my parents don't know, but this sophomore at my school somehow got a picture of me riding my neighbor, he won't send me the picture, but how does he even know about it? I know my neighbor has nothing to do with it, because why would he risk jail time by telling a 16 year old boy we had sex? He said that if to don't send him nudes and have sex with him, that he'll report us to the police and tell my parents. I'm so scared that he might actually have proof, if not how does he even know about us having sex? I told my neighbor and he said we should stop for awhile. I'm 18 in two months, so if I can stall the 16 y/o for that long, can my neighbor still in get in trouble if I'm 18?