Hi N..
My son had a similar problem when he was little, and occasionally still experiences it but not as bad. His doctor referred us to a pediatric urologist and we found out that he has a condition called bladder hyperreflexia. It basically means that his bladder thinks he has to go even if there isn't much in there. We did medication for a while and worked on retraining the bladder by gradually extending the time between going to the bathroom. For example, you said he goes about every 20 minutes, so try to get him to hold off for an extra five or ten minutes. Then each week try to add a few more minutes until you reach a more acceptable interval. Don't make him wait until he has an accident or anything, but encourage him to wait if he can. At first that was really hard for my son, which is where the medicine was helpful. Good luck!