Children sleep more deeply and soundly than grown-ups and do not wake up with the signals from a full bladder; so they wet their beds even though they may be dry by day.
Between two and three years, the sleep becomes lighter and the child wakes up and expresses the need to pass urine and does so only on waking up in the morning. Make him use the loo when you go to bed. That would be two or three hours after he has gone to bed. He will do it without waking up completely and continue to sleep peacefully after being put to bed.
Occasionally, night control does not start till four or five years. This is nothing to get worried about except that you have to continue with a mackintosh and plastic panties a bit longer, and have a little more washing to do. If you do not get too much worried about this, it will be a passing phase. But if you scold the child, beat or humiliate him, it could well develop into an emotional problem.
As long as the child has no urinary problem during the day, such as an unusual frequency or difficulty in controlling urine, there is nothing to worry about. If bed-wetting continues after the age of five years, you should consult your doctor.