Bsuttoi I recommend switching some stuff up.
I have had this feeling with girls I have dated before. Its because the sex gets boring.
Try dressing up in different stuff and surprising him. Try a school girl outfit or a librarian or something like that.
ALSO. I know this is gonna come off as crude, but it needs to be said. It may very well be the issue u are having and u dont realize it.
Do u keep up on your hygiene??? Make sure that your "private area" is fresh! If that shit smells bad then do something about it. Like right now if u play with yourself and smell your hand is there a unwanted odor on it?????? (lol...i know this is a bit over board..but I might be doing your bf a huge favor right now). If u smell anything fishy or anything unappealing in general, then I am willing to bet he is being turned off by that.
If thats not the issue then maybe he is just not sexually attracted to u anymore and its time to find someone who is.