@That-Guy Since you're not a close friend with him, he most likely would freak out at the direct approach. if you did get the chance to ask to masturbate together, I think your best approach to this would be to make it seem like a spontaneous thing opposed to something you've been planning. and it would have to be when you two are alone instead of with his other friends around.
Times have changed a lot since I was 15, You have the internet and many other digital means to stimulate your sexual psyche. boys are boys and when your 15 with a raging boner, you find a way to release that tension. I was always lucky to begin talking about girls, sex looking thru hard core magazines until I could see the wet spot from his boner and I would always start rubbing my dick a little thru my pants while looking thru the magazines making sure he could see my hardon too and then say I can't take this anymore. My balls are starting to hurt and I would start unbuttoning my pants and sticking my hand inside rubbing it. then I ask if its safe here to bust a nut without getting caught if he says yes, then pull it out and start jacking off looking at the porn saying outlaid what you would be doing if you were the actor and encourage him to pull his out too and bust a nut together. If he does, great! take it from there.. If he doesn't, make the decision to jack off so he can see or privately but follow thru. It adds to the whole scene. Play it by ear but if he's really against it. don't do it. That's kinda what I did when I was in my younger teens and it worked well for me. Good luck and be safe not stupid