Sounds pretty normal really. My oldest didn't start going through the night til about 5 1/2. He is 6 1/2 now, and still has occaisional accidents, maybe once a week. But not major ones, more like little leaks. :) My next little boy is 5 now (december birthday) and still wears his "nighttime underwear" We use cloth diapers, so it's the cloth equivelent of a pull-up. He wets them almost every night still. The nice thing about cloth though is that it is much easier to tell how wet a diaper is, and I can see they are getting less wet recently. With cloth they also feel more wet to the child, so they are more awre of it. We have gone to a bit less absorbent version and not having problems. i am hoping he will move toward dryness soon, but each child is different.
Now my daughter is 3 and she is already dry almost every night. She potty trained day and night pretty much at the same time, at about 2 1/2. she still has occasional accindents, but they are usually odd instances, like she fell asleep early and didn't go before bed, etc.
A very good friend of mine had a girl who didn't stop nighttime accident until she was about nine, then, all gone. It doesn't mean something is wrong with him, though of corse there are possibilities. It could just be normal for this boy.