That's the thing, hey. What causes constipation in one may not in another. Generally speaking, nuts don't really cause constipation because of the amount of fat in them.
How is he on MacDonald's? I know that cleans quite a few people out.
You could try a salmon oil caplet...pierce it open, and put it in some food you feed him. That might keep thing regular.
You can ask Dr. B what he thinks about that. I don't know if they smell...or taste bad, but for an adult 3-1g capsules a day goes a long way to prevent heart disease, too.