He's 3. Can't he have a drink of milk (if you like), or water, by the bed, and help himself?
he doesn;t need you to get him a drink. Perhaps that could be a bargaining tool?
I agree, if he sleeps during the day or if he doesn;t he's hell after 4, means he desperately needs more sleep at night.
He has got used to you going in - could this be for reassurance, comfort more than anything?
Does he take things to bed with him? favourite toys, comfort blanket, etc? I think you have to accept that he probably is going to wake at night but how can you handle it so he doesn;t wake you up as well!
Do they have a nightlight in the room? If you older DS sleeps well perhaps you could consider it? Perhaps he doesn;t like the dark.
I once went into DS at 2am when I'd been out late, and he was wide awake and shining his torch under the covers...
I don;t mind as long as he's not waking me up. I think this is rare but as I'm usually fast asleep at 2am, perhaps he often wakes up at that time!!