Actually, as many as 15% of children still wet at night several times a week at age 6, so don't be too concerned. It's really a developmental thing, not behavioral. My 6yo still has problems with this. It simply doesn't wake him up. He's a VERY deep sleeper. Every child is different, but if you were to talk to your son's doctor, I'll bet he would say that you don't need to worry about it until at least age 6. That's what they told me. We tried an anti-diuretic at night this year, but it didn't help too much. So we ruled that out and decided that it's just a matter of him not feeling it until it's almost too late (during the day still) and when this happens while he's sleeping, he doesn't have enough time to wake up and get to the bathroom. His nervous system is still developing in this one area and it's actually fairly common.
So anyways, my best advice is to not worry about it too much. When they're ready, they're ready. Other than restricting liquids and waking them up yourself to go potty at night, there's not much you can do. If those tactics still don't help, then you can bet it's developmental and eventually he'll be staying dry himself.
Best of luck!