M., i have no advice as i'm in virtually the same boat as you. so i'm kind of glomming onto your request. please forgive.
my son, who'll be 2.5 at the end of the year, can keep his 'car panties' (pull ups w/lightening mcqueen on them) dry all day long (even occasionally pooping in the potty), but as soon as you put underwear on him, he pees/poops every five minutes, so we put him back in the pull ups. (the reason we got them in the first place is b/c he wanted to wear his underwear right before we were going on a driving vaca w/over 18 hours in the car both ways - all my friends told me to avoid them).
anyway, what i don't understand is that he's obviously capable of holding it, so how could he not be physically ready? any thoughts, ladies? i do like the idea of the boxers if i could find any in his size.