I had planned to do the same as Clover and wait until they wake up dry, but when I forgot a nappy one night she just managed, and I now have to get her up in the middle of the night to wee...what a pain that is.
DD was nearly 5 after TTing in the day before 2.5.
It sounds like he's just not ready.

Oh and don't let him listen to anyone who tells him something detrimental about it....my MCHN told me it was highly abnormal for her not to be dry at 4 overnight....then I asked on here should've trusted my gut.