M.B.
Now, perhaps! To continue it to be "positive", just don't make a big deal of it if he pees while wearing underwear- that will keep things going well. Change him quickly and say "better luck next time!" or something like that and move on.
My kids LOVED picking out their first pairs of underwear at the store. One of them really took to a neat-o idea we had about underwear: we got underwear that had kittens on the front, and we'd say to her "try not to get the kitten wet!" And it seemed like she loved having an additional reason to not pee in her pants, she made it her main focus to not pee on the kitten. This same child was the type to ALWAYS forget that she had to go until it was too late- the kittens helped her remember.