Hi, I just read this and a couple of things spring to mind:
1) Children often revert to diapers (nappies), just when we think: great! He is trained now! Be patient, eventually he will get it.
2) Try a different potty (I assume you mean Toilet) seat. my 2.9 years old hates anything which is too small. He doesnt like conventional potties or toilet seats, as he likes sitting wiiiiide astride the normal toilet seat. (btw, they recommend all children, regardless of sex, being trained peeing sitting down. For a 2+ year old, peeing like a grown up, with no nappies, is a big deal. Dont make it even bigger of a deal by asking him to aim! Whenever my son goes to the loo in restaurants etc, where the toilet seat may not be that clean, I help him to pee standing up and he is petrified he will make a mess. if sitting down, I have to remove his shoes, even socks (!!!) trousers and underpants, so that he can sit astride. Pain, I know, but it does the trick .
3) For a poo (and I assume this is your biggest problem here), try and make time for him when he goes at home. I sit next to him, we talk, it is a calm time, you may try reading books to him too. mine loves this time sooo much, he makes a poo last up to 45 minutes, purely because it is fun! (a different problem altogether!)
4) He may be scared of the splashback. Put some loo paper down the loo, before he goes. This will cushion the splash. (sorry so graphic; that´s motherhood for you!)
5) Although it never happened with my son, I read in a few of the toddler books, that some toddlers are terrified of their own "body Product" being flushed away. Show him the poo (even if it is from the potty, being tipped into the toilet bowl), explain where it is going and wave goodbye. Yes, quite literally: Bye bye poo!!! (in a cheerfull manner)
6) Talk to the daycare. They may hurry the children (my son never goes there, he saves it up for home). They may have flushed the toilet whilst the child was sitting on it. They may force him when he doesn¨t want to go.
Then again, it may be just a whim, that your 2.5 year old is perfectly entitled to.
Best of luck, hope you found some of it useful!.
L. x