A.H.
If he eats well, try lots of fruits and veggies, make homemade fries- leaving the skin on- you can bake them in the oven- peanut butter on an apple- leave the skin. Salad- cucumbers (w/skin).
You could always ask the doctor about mineral oil, that has been used for many years and was suggested for my son who ate a lot of stirofoam once, I just don't know the proper dosage, but I added it to his koolaid, and he never noticed.
Good luck