My S.O. never wants to have sex anymore, and I want to know why!

My boyfriend and I have been together since February of this year (2017) so that's only 5 months and we've been living together for 4 1/2. Yes, I know that's early but I felt there were personal matters, and I let him move in with me. But like for the last 2 months, possibly more. He hasn't wanted to have sex with me. He's blown off all my hints, flirting, and put right put it out there to "fuck me" (excuse my language) I hate to think of the worse reasons, and excuses to why, but I want opinions. And I want advise on how to get him to have sex with me again, pleeeease!!!!