dd potty trained before big brother?

I posted here several months ago about my now nearly 4 1/2 yo ds who refuses to use the potty. Or even try to use. Or even go 5 minutes without a pull up. Well, now our 2 1/2 yo dd is using the potty like a champ, and I still can't get ds to show one iota of interest!

I know they're different kids, girls train earlier etc., but honestly, this thing with ds is driving me nuts. He's even getting worse about not telling us when he poops, and if he's out playing, he'll just sit in the stuff until it's all over the place.

Dh wants to force the issue by refusing to by diapers or pullups for him, but I'm worried that he'll get constipated and just turn it into a bigger power struggle. On the other hand, I feel like NOTHING will ever convince him to use the potty, and he really will wear them until high school. He's in a preschool 3 days a week which fortunately doesn't make an issue of it, but I feel like we can't move anywhere else (which has come up recently) because no other preschool will take a 4 1/2 yo in diapers.

Any advice? Or should I just continue to do what I'm doing, which is asking him periodically if he wants to try, but otherwise not making a big deal of it?