How often do you do it? Maybe you should give it a while. If you constantly ask for it, it gets old and he gets annoyed. Gum up the works. Buy the end of the month make him want it. Dress up. Make yourself look really hot and then just love him like that instead of wanting sex. Eventually he'll get horny and think, "well it has been a while she hasn't bugged me about it or anything."
I'm always wanting to do it. My boyfriend and I have been together for about 4 years. Eventually he started saying he was to tired and that's all you wanna do. I waited patiently. Did little things in front him, got him to laugh and take bubble baths with me(without sex). I dressed up everyday(not in costume or anything, just as myself, hair fixed, new clothes, all that good stuff) and he started getting horny. Wanting to do it. So we finally did and just like you said above, he only lasted 5 to 10 minutes. Well that's the good thing about round 2 ;) and three and four haha. I always f*** him. I don't want him tired not this early. Say naughty things like "uhhh, make me cum", or "you like that wet p****", etc, etc. Be really comfortable. Every time my boyfriend and I have sex its good, even if it doesn't last long because I love him.