K.T.
Hi, I agree alot with what CR has to say here below.
I am having the opposite problem with my son---he does great at daycare but then doesn't want to sit on the potty when he is at home. I think the routine at daycare and all the other kids participating helps him follow along.
I know at my daycare they will participate in potty training if and only if the parent is doing this seriously at home. Otherwise they "offer" the potty to the child. Even if they are potty training the child along with you, they still cannot make the child pee on it if they do not want to use it. And if your child doesn't want to use it at the potty time, they also will not make exception for your child to put them back on alone in another hour, but at the next group potty time. That said, they also do not let the child sit in dirty dipes if they are messy, that's common sense and decency to keep the kids dry.
Try working with the daycare more and find out their routine and see how closely it matches yours. Kids also will start to potty train and then will start to balk, and go back and forth for awhile (2 1/2 is still early for most boys)--- so I think putting him in underwear to force it wouldn't really help anyone. (My son one day hated the underwear because he felt pressure---so we had to stop training till he got comfortable again.)