I completely understand the not wanting to go back to diapers thing. Thoughts on a waterproof pant (or diaper) over his underwear or pants (& lots of changes of clothes, of course)...that way, you are not going against the routine at home and still saving the clean up at home. I too always explain to parents how much harder it is at daycare...they are often distracted in play; sometime they feel ownership over the toy they're playing with and don't want to leave the toy to go to the bathroom, we are busy and easily miss the cues/reminders....it goes on and on....
I wish you luck!
Once thing for sure...until the little one are completely ready to cooperate, it is all quite a struggle.