What kinds of fruits does he eat? And do you give him anything - ANYTHING - with artificial sweeteners or sugar alcohols (splenda, sucralose, aspartame, sorbitol, xylitol, stevia, truvia, agave nectar, etc...literally anything other than sugar)?
The reason I ask, is that some people are extremely sensitive to certain fruits, and it can produce the opposite-than-expected result. My daughter cannot have sorbitol (or the other sweeteners ending in "itol"), or the fruits they naturally occur in (apples, cherries, purple/red grapes, pears).
A rather simple experiment would be to eliminate all apple, red grape, cherry and pear products from his diet for a week or two (maybe a month). Let him have strawberries, green grapes, blueberries, bananas, oranges. Check every juice and drink label (most juices and kid drinks are sweetened with apple juice or pear juice). Give him 100% white grape juice if he needs a fruit drink, or water, and check that it's not "white grape juice with other natural fruits". Remove all sugar-free foods from his diet (most are sweetened with sorbitol or the "itols").
When we removed all sorbitol and apple/pear/cherry/red grape-containing foods from my daughter's diet, her constipation, which was so serious that her intestines were beginning to atrophy and they were considering surgery, became controllable, and improved remarkably. Her GI dr explained that many kids are very intolerant to the sorbitols (used in sugar free foods and naturally derived from apples, red grapes, cherries, pears, some seaweeds, and a few other sources that aren't so common). Some kids react with constipation, others with diarrhea, others with abdominal pain. It's a fairly easy thing to eliminate and try.