Hehe, it's not our fault. It's called the refractory period and is completely chemical and biological, not our conscious choice. After ejaculation and orgasm we lose all arousal, "horniness", and erection and until that period ends we're really not going to be able to, nor are we even interested in having sex again.
That said...
If he's too tired, then do positions where you are the one responsible for movement. But I'm guessing you mean, he's tired as in sleep, not tired as in can't move anymore. Either way, if you're the active one, he might have fewer objections to having sex more often and longer.
Also, if you're desire is to ejaculate from actual penetrative intercourse, perhaps you could get yourself closer to the point with foreplay, playing with yourself, etc. beforehand. Then you can get off before he tires out.
As for decreasing the refractory period, there might be some things that can help. Every man is different, so it depends. Some guys are good in a few minutes, others take a considerably long time.
I find that although I'm no longer horny, I can be quickly turned back on with continual playing. That doesn't mean stimulating my penis. In fact that wouldn't work at all, because it's sleeping and doesn't care who is knocking or how good they are at knocking. However, during my refractory period I continue to please my partner in other ways. Usually with my fingers which quickly gets her moaning and squirming.
And it is that which ends my refractory period. I'm not thinking about sex. I'm not thinking about getting another erection. I'm not getting physically stimulated by her or myself. I'm not thinking about having another orgasm for myself. I'm just working on her. But once she displays her extreme pleasure through moans, squirms, pulling me closer and kissing me, breathing hard into my mouth, etc. I am instantly ready to go again, because it turns me back on.
But that's just me. Her moans turns me on more than her touching me. It might be different for your guy. You might have to touch him to get him ready again. Or maybe if he's too tired to touch you, you touching yourself while he watches and listens will be enough to turn him on. Look into his eyes. Look at his penis hungrily. Talk dirty. Whatever you have to do to turn him back on.
I'm sorry, we just turn on and turn off faster. So you might have to put in a little more effort. Hopefully if you do, sex for both of you will be that much more enjoyable and then you can work on being equal givers and receivers more easily. Finding what works for him and doing it may seem like you're giving more at first, but it comes back around to you.
Or like the other poster said, try a "horny diet." Not sure what that is, so ask her. But perhaps some of those herbal viagras might work too. There purpose is generally to create harder and easier erections, but I think they might also increase libido.
Sex is fun, but is not always gonna be the best. 6 hour sex nights are great, but can't be done every time. So experiment, have fun, and try new things in all different categories.
Do you have any difficulty achieving orgasm?