Drinking extra fluid will only make a difference if he is taking something like lactulose. Otherwise he'll just pee more!
What sort of diet does he have? We found that certain things would cork Abby up, bananas were the worst, and white bread too.
We would give Abby prunes once a day and that kept her moving. Lactulose is also a good thing and is available on prescription. You can also try giving some fresh orange juice (diluted) We gave Abby some diluted fruit smoothie the other day and oh my word she moved well for a few days after that!