M.F.
I don't see a problem with making the switch to a pull-up at night time only. Our pediatrician actually said that they don't expect full night-time dryness until age 4! I would just praise him for his efforts duing the day and start calling his pull-up or diaper his "night-night" pants or something to make the distinction. If you haven't done pull-ups at all yet, you could get some now and help him understand that they are only for sleeping in.