My boyfriend has a very low sex drive and I can't take it anymore! What do I do???

My fiance and I have been together for almost 4 years. We've been having sex for about a year. Within the last couple months though he doesn't seem as into sex; He never even atempts during the week. We have sex maybe once or twice on the weekend. Sometimes not at all. We've talked about it and he says that it's not me at all and it's something with him but he doesn't know what. He's always tired so I tell him to take a nap after work. He says it'll help to change things up but he doesn't give me the chance to. He'll send off signals in the evening but then fall right to sleep when he is hits the bed - or couch. He's not like your typical guy wanting sex all the time - sometimes I feel like the guy which is not cool at all! Sometimes I just want him to take charge and be an animal but he tries to be "gentalmanly" about asking for sex and so sometimes things are so slow to start up that HE falls asleep! Is it normal for a guy to be told he’s going to get sex tonight and tell you no because he’s tired and we don’t have anymore condoms? A store is right down the street! I love this boy with all of my heart but this is starting to get to me….I can’t handle the confusion anymore!

Please help me/him!!!