I agree with Dana. My son is turning 4 this month, and still wears a pull up for sleep. He has been totally potty trained during the day since around 3, but there are still mornings when he will wake with a wet pull up. He will not pee in the pull up if he is awake, he takes it off to pee, and if it's night time, he puts it back on. If it's day time, he will get undies and put them on. We make no issue of this. I totally believe that if it's happening in the middle of the night, he CAN'T control it, therefore, it would do no good to have him go through peeing in undies, wetting the bed, and going through that routine.
I wet the bed well into my early elementary years, and I promise you I had NO CONTROL over it. There was NO behavioral component to it. I just didn't wake when I needed to go. I wish I had had pull ups or the night "diapers" for older kids. It would have saved me a lot of embarrassment as a child. Incidentally, it did subside sometime during elementary school, so it doesn't last forever.
SOOOO, I feel strongly that you should just allow your son to develop naturally at his pace. And realize that it may be sometime before he is able to consistently wake when he has to pee. It may happen sooner for him, but just realize that if it doesn't, it doesn't necessarily mean that there is something behavioral that's going on that you can change. It may just be that he's physically not able to wake in the night when he has to pee.