Hi. Insread of one of the water drinks, try 1 oz fresh orange juice with 2 oz cooled, boiled water.
It makes my little one go everytime, and has helped out a coulpe of others on this website.
If it still doesnt work, go to your Drs and get some Lactulose. (It reabsorbs water from the intestines to make the stool softer. It takes a couple of days to work and once established bowel opening has started, you can wean him off it. You'll probably be prescribed 2.5mls twice a day, so cut it down to once a day for a few days beofre stopping. It just allows thier gut to do the work instead of becoming reliant on stimulation.) My daughter needs the regieme every few weeks, but lactulose has no other side effects/long term issues.
Good luck! I know how frustrating it is!