Here is what we did, which worked for my son. He is a VERY sound sleeper. We waited until he was almost 4. Then we invested in several washable hospital-style incontinence pads for his bed. We put him to bed in only PJ's, no diaper, no pull up, nothing absorbant. Before going to bed and without trying to wake him up, one of us would scoop him up, carry him to the bathroom and put him on the toilet. (If we tried to wake him up he'd have a fit and not sit down) He would always pee. After a few weeks of us scooping him, he would start to wake up fussing, we would carry him in and he'd pee. This evolved into him running to his bedroom door and not knowing what to do, so we would lead him to the bathroom and he'd pee. After a couple months he started going into the bathroom by himself, and even remembering to wash his hands and flush. In conjunction with this, we did a sticker reward program for dry nights and he earned a couple Star Wars Lego sets over 6 months.
He had very few accidents, but if he had an accident we'd strip off the wet incontinence pad and put a new one down, change his PJ's and he'd be asleep again quickly.
He is 6 now and still gets up to go to the bathroom almost every night, sometimes twice. He has been accident free for over a year. So even if they're bladder can't go all night, it is possible to train even a sound sleeper to be dry at night.
As an aside, and I'm sure you already know this, but even if you were able to get him totally dry at night, don't be surprised if things change and he's no longer potty trained when baby #2 arrives and his world gets turned upside down!