Daycare has an understandable policy that a child must have two weeks in a pullup without accidents before they can wear underwear. Not a problem, right?

Except now my 4 year old believes pull ups are just nicer diapers. He refuses to use the toilet at daycare (and has for two months now). We did boot camp over the holidays and he was totally potty trained at home. After two months of trying to switch to underwear when we get home and on the weekends it has become clear he is regressing. He had three accidents this last week because he just forgot he wasn't in pullups like he is ALL DAY at daycare.

I can't take two whole weeks off of work to give him two weeks without accidents, the pullups clearly aren't working and daycare won't let us bring in a little potty for him that he is willing to use.

Any ideas? His little sister is almost 2 so I'm wondering if she's trained if that will get his butt in gear, but I don't think he actually cares that much.