5 pairs of undies, 5 pairs of pants, and crossed my fingers!
He did great this weekend at home, but we needed to remind him every half hour - he never once told US that he needed to go. He had bunch of accidents on Saturday, but only 1 yesterday and I just didn't see how sending him to school today in a diaper would not be confusing!
I called at lunch time - about 4.5 hours in - and he'd only had one accident, and it was a poop (something he's not done in the potty chair at home)!
We are planning a road trip in 2 weeks, though... any ideas how to cope? It's 6 hours each way...

Re: Sent DS to preschool today with...
In 2 weeks you'll be amazed with the progress if you stay consistent. My DD was fine with pee after a day as well and took about a week with poop. . . and then all was well.
I did keep a waterproof pad under her in the carseat (one of those disposible ones. . . just cut a slit for the crotch strap to go through) for my peace of mind for a for a few weeks.
We were out and about doing all our regular stuff after a week--I just made sure she went before we left, when we arrived home, and every 2-3 hours while we were out.
And you know, she NEVER had an accident in the car anyway. I'm sure if I hadn't put it there, she would have
You are way braver than I am. I would not put my kids into underwear until they were able to poop and pee on the potty and they were telling me they had to go. older DD was trained at 2 years 9 months and my younger DD will be 2 in a few weeks. She pees on the potty about half the time, always tells me when she has to go on the potty or after she has pooped in her diaper. She refused to poop on the potty and I will not let her wear underwear until she starts too.
As for the car ride, invest in a travel potty seat and be ready to pull over to the side of the road at a moments notice and let him go potty.
WHY didn't I think of that??? Claire peed once in her car seat and it was while we were on a road trip...luckily it happened so quickly that I got her out before the seat was wet. She had just gone a little in her undies!
I suggest pull-ups for traveling. I just made Claire wear one last week when we flew, and she's been potty-trained since June!