Dirty pants problems

[swg1]

My son is three and has been attending the same nursery since he was just over one. He currently goes three mornings a week, and has always seemed really happy there.

He's recently moved up to the preschool room. Keyworkers there seem really nice, he's always happy and playing when I pick him up (and a few times has protested being picked up). There's just this ongoing issue with pants.

He's potty-trained, but in the 2-3 room they were usually escorted to the toilet. Now I think he's expected to go on his own and he's not wiping properly. He's regularly coming home with a really dirty bum and I'm worried he's going to get sore. I've talked to him about it, talked to nursery about it, asked nursery to check him, practiced wiping.. and today I got to nursery to pick him up, sent him to have a wee, and he came back out to tell me he's had an accident. Except apparently he had it at lunchtime, about an hour before, and didn't want to tell anyone but me he had dirty pants.

I don't know what to do. Adding to stress, we really need to change him from mornings only to full two days in January because my working pattern is changing, but now I'm worrying he'll be sitting in poo for a full day.