Same situation...3 year old who has been trained during the day since 2 yrs 2 months, still won't wear underwear at night, although he is ALWAYS clean and dry in the morning (for nearly 6 months). He sleeps for 12 hours, so I am not pushing the underwear...Also have a 5 week old...and I want to sleep!!!
Two ideas.
1. Wake him to go to the bathroom before you go to bed (or when you wake up to feed the baby - if he doesn't fully wake, then he may be fine going back to bed.
2. My son knows his numbers - don't know if your son does or might be able to learn them. We have a digital clock in his room and tell him that he is not allowed out of bed if the first number is not a 6 (or sometimes a 7 if I think he is really tired). He can play his music (which he can turn on by himself) or read a book or play with his animals, but he can't get out of bed...He doesn't have to pee immediately in the morning (a camel or something) so that might not work for your son, but maybe...Also, we got him a digital watch which he thinks is amazing - so he really likes numbers...