Night training is a misnomer and a whole new ballgame, you can't "train" your son to do it, he'll do it when his body is ready. Some children, boys and girls, can't physically stay dry at night for many years due to immature bladders and not waking before it happens (hence the thriving "big kid" overnight pull-up industry) and doctors typically aren't even concerned until a child is at least 6. Each child is different, even within the same family, so try not to compare.
Make his bed up with a large waterproof pad, a bottom sheet, then another waterproof pad and bottom sheet, so if he wakes you during the night you simply strip off the top wet layers and he has a fresh bed to go back to sleep in. Keep having him go potty right before bed, keep a potty chair in his room, and a night light, and encourage him to get up on his own and use the potty if he wakes during the night. He'll get there eventually.