DD1 got toileting pretty quickly, would take herself to the potty unprompted and no accidents, but like your boy, it was as long as she was not wearing any pants. We had two or three attempts at knickers at home, each ended in accidents everywhere, washing for days and me going nuts, so we went back to pants off.
After a while, I decided enough was enough, that she had to toilet train with clothing so I started reminding her every so often instead. Since she knew her cues, it worked well - I think the issue here was as soon as there was clothing of any kind, she felt she was wearing nappies so got lazy (she had worn cloth nappies so perhaps the clothing felt similar? Or maybe just having *something* on in general?). We are still dealing with accidents here and there - fantastic with poos, a little lazy with wee's if she is zoned in on something (usually tv, my couch has taken a beating!)
He will get there eventually and is still quite young so try not to stress - perhaps a different approach would help though (i.e. the reminding/asking if he needs to use the toilet, and getting daycare to follow suit). I would be inclined to focus on getting him into undies at home in the first instance, which will hopefully have a positive effect on him at daycare too. Good luck