Do you mean that he falls asleep instantly, before even rolling back off you? Or he rolls over and falls asleep soon after?
In general, orgasms make men very, very tired. I have to be very excited not to feel overly sleepy after I cum. And generally, sex for us happens right before bed time, so my body is programmed to want to sleep at that time as it is. But it doesn't happen so fast that I can't roll over first and snuggle a bit.
So, further explanation of what exactly is happening will help us (me, at least) determine what might be the best way to help.