Not sure what country you're in but if you're 15 currently I'd be more concerned about your relationship. That is, if the legal age of consent is 16 as regardless of the morals around a relatively small age gap in the eyes of the law that is pedophilia. I anticipate your friend is OK seeming as his boyfriend is already at the age of consent.
A tangent: a 15yr old friend were caught getting frisky with their 15yr old boyfriend and they both got put on a sexual offenders list! Madness