I have a friend that lives in Alaska who had the same kind of situation. The result of her situation was pregnancy, so I would rethink your situation before you make any moves. She's 19 now, and he's 26. That's 7 years apart. She was pregnant at 16. AT 16. I don't know this guy so I can't make any judgement saying that his only intention is SEX, but it is normally the case when it comes to that big of an age difference. I get that you might think you're mature enough for him, and that you can handle yourself, but please be careful. I will say that they are together at the moment, but they have been through so many breakups and are barely together now.
Honestly, it's just bad news to be involved with somebody that old at your age. I can understand your desires, but think of this logically. You're 15, he's 22. Men at this age are driven towards pretty much one thing, and I'll let you guess what that is. Like I said, MOST, not ALL. Again, I don't know him. If you guys have already had sex, and it's been less than [let's just say a year], but a few months, I would say that's probably one of the main reasons he's involved with you.
Please, for the love of GOD (or pancakes, whichever you prefer), THINK. I understand you're lonely, I used to be the same way, but THINK.
Good Luck!