We tried prunes, apple juice, grape juice, pear juice and all the fruit my son could eat (and he can eat a lot!) and it didn't take care of the constipation. If your son eats well and drinks lots of water like you said, he may just be a little slower. Some people are just like that. You don't want him to be uncomfortable.
Our pediatrician recommended Miralax and after putting it off and trying a lot of other things, we tried it. It works great. It actually worked so well that I felt really bad that we hadn't tried it earlier. I hate to think of my little guy and what a hard time he had until we started using it.
According to everything that I could find on it, it isn't addictive, unlike fiber laxatives. It pulls water into the stools and makes them softer and easier to pass. It doesn't cause cramping, thank goodness. It really worked wonders with our little guy. After we finally used it, I talked to a number of other moms who said that they'd used it for their sons. One of the moms said her son used it for several years before he finally became regular on his own. Maybe boys just have this problem more often. Who knows. But Miralax has been great for us.