Can my mom go to the police on my 21 year old friend?

Your mother could threaten him with statutory rape but you say you haven't had sex. When I got married at 17 (2 months before I turned 18) the cops told my mother that it would take so long to come to trial that I'd already be 18. Your case is probably the same. Here's an idea. Put him on hold for 5 months. If he really loves you, he'll wait...