DS started potty training but would not use the potty at preschool--how to handle?

[HIU8]

Yesterday DS woke up and said that he wanted to use the potty only. So, we started potty training. We spent yesterday naked potty training and DS went pee 4 times with no accidents. No poop yet, but I expected that. Anyway, he wore a pull up to preschool today, but would not use their potty because it doesn't not look like his. They won't allow us to bring a potty seat to go on top of their children's toilets. I'm trying to convince DS to just use their potty. He held it in for 2 hours today instead of going. He gave up diapers yesterday because pull ups are for big boys according to him. Anyone BTDT with preschool? Do your DC eventually just go at school also?

Thanks.