There is no way around it, if he is having accidents during the night, he should go back to pull-ups.
This happened with my DD. She was potty trained at 3, but continued to have accidents at night. After about three nights of me getting up in the middle of the night to change her sheets (one night 3x's), I was exhausted, and told her that we had no choice. She cried and was upset, but I assured her it was not a punishment, but that Mommy just could not keep changing sheets at night--it made me a grummpy Mommy during the day! I think putting the burden on me helped. She then wore them until about 5 1/2.
Luckuly she didn't do sleep-overs until she was 6. But, would he rather have an accitent at a friend's home, or wear a pull-up. No one needs to know.
I wish you luck-- I know this is so hard on their little self esteem. This too will pass:)