Hi Katie,
I've been a licensed home childcare provider for over 20 years. In my experiences with potty training, it sounds to me like it's mostly turned into a power struggle for everyone. However, since your son is about 6-8 months behind due to his speech & language delay, my thoughts are he needs a little more patience & understanding from his caregivers. I agree, you should insist that he should wear his underwear at daycare. In fact, it disturbs me a little that daycare puts diapers on him when he wets, as I feel this is only their convenience. I'm curious if he wets himself at pre-school as often as at daycare. I suggest everyone to relax and not make a big deal out of it (especially to or in front of him). As a caregiver in my daycare, I tell my little potty trainer, it's time to go potty (sometimes I have to tell them several times, as they're busy playing),eventually they run into the bathroom (all by themselves), and do their pooty thing, sometimes they make it and sometimes they don't. If they don't make it, I simply tell them to get some clean underwear and pants out of their bag, and change your clothes. And that's exactly what they do, no big deal! Eventually, they get the hang of it and before you know it, they're running in the bathroom all by themselves. Until then, make sure you provide, lots of dry underwear & pants for him to change into!! Good luck!