If it seems like he is not in control of this habit, just hold steady mom and love him through it. My four year old daughter has regressed back to night-time wetting since our baby son was born. We use Goodnights (they are like pull-ups) for her at bedtime. They are great at catching a large amount, and saves me having to wash sheets every day.
If you think he is taking advantage of the diaper and using it instead of getting up to use the bathroom....My second son did this, so we took the Goodnights away and if he wet, he was wet. If it was the middle of the night, we'd lay a towel down. So I was changing sheets everyday, but he really didn't like being all wet, so he seemed to make an effort to stay dry. This lasted maybe two weeks, but then we had it down, with occasional accidents.