If you do add benefiber (it is a good choice) make sure he’s drinking lots of fluids as well. Since it’s a bulk forming laxative, it needs fluids to keep the stools soft.
All bran now makes great tasting “snack bites” – seriously, they taste pretty good! I buy the cinnamon and brown sugar flavoured ones. I got them for myself, but my 2 year old begs me for some whenever I eat them!
Raisins also have lots of fiber, and kids usually like to eat them better than they’ll eat their veggies!