Have you ever given him an enema? Not the Fleet chemical disposable type, but an old fashioned one using an enema bag? Enemas are often recommended as the first step to cleanse the bowel prior to beginning a bowel retraining program, and an enema is the quickest and most natural way to produce a liquid stool.
Many parents are reluctant to give their child an enema the first time, but an enema need not be traumatic or overly uncomfortable if it is done properly. If you are interested in more information, I'll be happy to provide it.
In any event, I wish you success in your efforts to help restore your son's natural regularity. It's something that is much easier to correct now at his age than it will be a few years from now.