Dear H.,
Just to let you know, nighttime dryness takes much longer to achieve than daytime dryness. My daughter was 2 1/2 when she caught onto going potty during the day, and she was 4 1/2 before she could stay dry all night. It is not your son not knowing what he is supposed to be doing, it is his body catching on and neither he nor you have any control. And doctors are unconcerned before age 6.
At least four things have to happen before he can stay dry at night. His bladder needs to be big enough, his body has to produce enough anti-diuretic hormone so he can stay dry all night, and the urge to urinate needs to awaken him so he can do to the bathroom. If he is a very sound sleeper, the urge will not awaken him and he will wet the bed. Scientists have tested children whose parents claimed they were very sound sleepers and found this to be true. The kids wore earphones to bed, and the scientsts sent a loud noise through the earphones. The sound sleepers slept right through the loud noise.
Some people claim that the pull-ups just encourage the child to keep wetting themselves at night, but they don't have to change the sheets every morning (and I didn't find it to be true in my daughter's case). Until the anti-diuretic hormone kicks in and his bladder gets big enough and he can wake up when he needs to pee, I recommend that you keep using the pull-ups, to save your sanity and your son's. I'm sure it is much more expensive to wash sheets everyday than one pull-up per night. If he doesn't make too much of a ruckus when you take him potty before you go to bed, keep doing that. He'll start to learn how to get up himself. Make sure he doesn't drink anything after dinner, too.
In the bad old days, I think parents kept their kids dehydrated so they wouldn't wet the bed (I know my mom did this with me), which could explain why nighttime dryness seemed to be achieved much younger. But we know better, let your son drink what he needs during the day, but cut him off after dinner.
Check out the website www.childdevelopmentinfo.com/disorders/bedwetting.shtml That is were I got my information on the mechanics of bedwetting and why it may seem to take forever for a child to achieve nighttime dryness.
Hope this helps. I know of perfectly normal, intelligent people who wet the bed until they were ten. It really is a developmental thing, That's why Goodnights were invented.