Have you given him enemas? No matter what cleansing program you may use for him, I would think the first step would be to get his colon completely cleansed.
How much does he weigh? A safe amount of water to use is one ounce for every 2-3 lbs of body weight. So for example, if he weighs 40 lbs, that would be between 13 and 20 ounces, or about a pint or so.
Go slow and be gentle. It may take several enemas over a few days to get him cleaned out.