M.D.
I say go for the underwear. If he has been dry for a week, be done with diapers. I let my dd stay in a night diaper for way too long and then it was hard for her to "night train". If he has got it, then go with it. If he has an accident, don't worry just let him learn in underwear. When my ds starting potty training, we did not use night diapers at all, he learned it all at once.
Good Luck!