pants soiling in older children

As far as the bed wetting goes he is small for his age and the doctor did perscribe a pill for him to take before bedtime to replace a hormone that reduces urine production at night. When he takes this pill he does not wet the bed, so hopefully as he gets older and this hormone develops he'll stop wetting the bed. We do not give him this pill every night only for sleep overs since we do not want to inhibit the natural development of this hormone.

The dirty underwear is another problem. We can't seem to get him to realize that when he has the urge to go he needs to stop what he is doing and go. And to take the time to wipe properly when he does go. All his underwear has stains and I have found feces on the floor in the bathroom on several occasions, that I think feel out of his underwear because he didn't make it to the toilet on time. At the begining of the school year he had 12 pairs of boxer shorts, he now only has 4. He claims he doesn't know what happpened to the other 8 pairs, but his father and I believe that he had accidents in them and threw them out.

I'm extremely frustrated by this situation so any help you can give us would be greatly appreciated.