Why doesn't he want me anymore? Please help!?

My boyfriend and I have been together for 6 months now. I am 24 and he is 26. I love him more than anything. About a month or two ago, he stopped being romantic, i felt unloved & unappreciated. So i tried harder, giving him little presents, back rubs and stuff hoping he'd be his sweet self again....this didn't work plus he stopped wanting to have with me... before this, he would be so sweet & attentive. we'd have for hours, a few times a day sometimes, & it was amazing. This month we had twice. I tried to intitiate it many times & he always has an excuse (sick, tired..etc). i can't take the rejection, it hurts so much & i end up crying myself to sleep. i tried to talk to him about it.. he says he is attracted to me & wants to but has no drive. he has a bad back for years & has been on painkillers for years & he claims that's why, although that never happened before... quite the opposite, they made him horny. so i don't get it or know what to do. i love him.. help me please.