I agree with M2FM. Bare bottoms aren't an issue in my house. Potty Training is hard work and very time consuming constantly watching the individual who is training. I would just be reminding him more frequently/requesting him to take a potty break during playtime to reduce the frequency of accidents. To make it harder for him to remove his underwear could back fire and make him have more accidents as when he needs to go he may not be able to remove them quick enough.

Koodos to you for being able to train a boy this age.If you still don't want him bare bottom and If he was a little older, and obviously I dont know his level of communication skills and how much he really understands when you talk to him, I would take him out and let him pick his own underwear and explain that he has to keep these on while the other kids are in the house. I would talk about it and answer his questions of "why?" until he understood. Maybe even offer him a reward system like stickers for keeping them on and tell him if he keeps them on he can reach into the mystery bag(pillowcase?)and the end of each day to pick a suprise pair for the next day. He might find this exciting and you could make it into a game and make a big fuss of the underwear.

Those are my suggestions and good luck