First and foremost, seek medical attention to make sure there is nothing physically wrong with him. Secondly, use pull-ups every night to assure he is not humiliated in the morning by a wet bed.
I have had several children in my family that were "bed-wetters". There were no physical reasons for these children to wet the bed. You could do nothing to make them stop, not chastising, taking to the bathroom in the middle of the night, withholding fluids, I really mean nothing worked. However, one night each of these individuals just quit wetting the bed seemingly for no reason and had virtually no "accidents" after that night.
One child even had surgery that did virtually nothing after about 6 weeks. This same child also had the immediate need to urinate, much like your son. No gradual urge, just gotta go now. She was probably 7-8 yrs old before no longer wetting.
I think something in these children is not developed enough to distinguishd a slight urge, therefore by the time it registers they need to go, it is a crisis. I would encourage you to help him by recognizing his immediate needs until his body and brain work together better. I suspect if there are no medical problems this will work itself out. Try to be understanding and patient with him and know he would not do this intentionally.