Going to highschool in the fall with bf, he's acting strange...?

Me and my bf have had a off an on relationship since i was 12 and he was 14.. at first it was stupid, just little kid stuff, but after a while i thought i might have found something that could maybe be really meaningful. im really close to his family, especially his little sister, who is my age. we went to different middle schools and met thru church. now that im going into highschool (were going to the same one) in the fall (im 14 now, hes 16), im really excited that we're gonna be spending way more time together. problem is, he has been acting really strange lately. we only have a few weeks til we get back to school, and he hasnt answered the phone in almost four days. he was being kinda distant before when he first found out that i got into the highschool (its a competitive school with acceptance by application--english isn't my first language,but i'm really good at math), but now things have really changed. i have a feeling i know where this is going, but im really, really saddened by it. i absolutley love his family and i dont want things to change! is there anything i can do? why is he acting this way after (the greater part of) two years being together?