just a tip - in the middle of the night, you may find it easier to change his wet clothes before you feed him - it may be that the feeding him relaxes him more than the changing.
and to answer your original question, maybe around 2 months, start to try & set his internal clock by starting around the same time everyday? really, you can start as soon as you want - but it's not very easy until they can start to do a 5-6 hour stretch at night! :wink: