We've gotten dull...

My bf and I have been together for almost a year and a half, we're both 18 and when we started dating we couldn't keep our hands off of eachother. However, these last few months sex has been almost non existent and I try to start things all the time but nothing ever really happens. I'm worried he's over me in that way and wants more of a friend than a partner. To me sex is very necessary in a relationship and I'm starting to feel like we're just friends. Any advice or similar stories?