I would try the Prozyme you would be surprised how great this stuff works especially since you have ruled out all med causes. I am not a fan of grains, my dogs get a grain free raw diet. But if I am going to feed a grain I find brown rice to be the lesser of the evils.
I do supplement 2 of my dogs with a grain free kibble by Timberwolf organics. The third is strictly fed a raw diet. Supplements I use are Canine Complete by the Wholistic Pet, Olewo carrots, salmon oil. Every few weeks I add some pureed veggies and raw egg. I also sometimes add canned pumpkin it is good for firming stools as well as for constipation.
Also you may want to increase his kibble by a 1/2 a cup or so on the days he is working, siince he is burning more energy on these days his body needs more fuel. Just some thoughts!