Holding Pee - Advice Needed

DS turned 3 in March.  We've done underwear/potty time over the past few weeks and knew DS would just hold it until potty time was over (defeats the purpose).  We went cold turkey to underwear or "bare bottom" time at home this past Monday.  DS TOTALLY understands and likes pooping on the potty.  That transition has not been hard, which from my understanding is typically the more difficult part of potty training.  He also can control his bladder.  He doesn't pee in the pool at swim lessons, nor does he pee in his bed at naptime (he wears a nighttime pull-up for overnight).

HOWEVER, whether out of defiance or whatever the reason, he is making the decision not to pee on the potty.  We've used incentives, we remind him every 20-30 minutes and take him to it, we've talked about it, we've had him help us clean up after all his accidents, etc.  He explains to us how great it is to pee on the potty, and how that's what we're supposed to do.  He totally gets it, and has done it, so it is not for lack of understanding or bladder control.

What strategies/techniques can we use to get him peeing on the potty???