If he has solid stool on the current food that's a good thing. It could be you're feeding too much or just too much per feeding. Try feeding the same amount but split up into three meals. Or it could be like you said ,too much fiber.
If you do decide to try another food. My dogs eat Purina Pro...
Victor is great food. I fed it to my dogs for a short time but their stool was never solid. I think the food is very rich. I now have them on Purina Pro Plan Sport 30/20 Salmon Formula. They have perfect solid stool every time.
Our dogs are eating Pro Plan Sport 30/20 Salmon Formula. They are doing great on it. It's 496 kcals/cup. It helped out Lab puppy gain weight.
We started with the Chicken PP but our dogs like the Salmon Formula much better. Maybe try that..