You guys are in high school. How many people that dated in high school end up staying together? The odds are against you because you both have a lot of growing up and life yet to experience. Do what's best for your futures and careers, NOT your relationship. If it was meant to be you'll find yourselves back together after college.
I'll be honest I think you two just need to give it up. If it has come to an open relationship it appears to me that you two don't seem to be making each other very happy. Oh yeah and btw even though he says he isn't hooking up with anyone he probably is and the moving 3 hours away should be telling you something. You stayed close for him and he can't do it for you? Seems to me like the relationship isn't as strong anymore.