I know exactly how you are feeling!! This may sound gross but it has worked for me twice. Make him wash his underwear out by hand.
My oldest daughter wet her pants once while she was sitting in the sand playing. She said she was too busy playing and didn't want to get up. We went straight home from daycare and straight to the bathroom. I mad her dunk them in the toilet to get all the sand out. She is 18 now and still remembers that. She says that is why she never wet her pants again.
Last week my youngest, 4 1/2, was at a playground and pooped in her pants. I told her on the way home what she was going to do. She really didn't believe me until we went straight to the bathroom. After she dunked them in the toilet a few minutes to get the biggest part off I made her scrubb the rest out in the bathroom sink. Then I made her clean the sink out. Later that night she promised she would never used the bathroom in her pants again, unless it was an accident.
Good luck!
R.