Not something we normally complain about...

My boyfriend and I have been together for just a little over half a year, I'm 18 and he's 17 but surprisingly mature for him being the younger one, recently I ran into a little problem with him sexually... I was attempting to give him a bj and I just can't do it for very long because my jaw gets sore from how wide I have to open my mouth to fit him in and my back molars still scrape him and I feel so bad! I know I'm still a little young for this kind of stuffs and him and I are moving a a bit quickly, but anyone have idea to help me?