Consider cutting back on the green beans. Yes, it could be that the exercise increases the frequency, given his age. Or it could be all those beans. The Metamucil may be easier on his system.

Thank you, Charlotte. I appreciate the reply. The beans were suggested by the organization that trained him. It's a way to fill him up with causing weight gain. One can may be too much at his age, I guess. He's about 73 pounds and weight is not a problem.

You an also try adding a little baby rice cereal to firm the stools up a bit, a forkful of canned pumpkin also works to firm up and soften stools like the Metamucil , but can be a tasty treat. Mineeat it right off the fork.
I did try the canine probiotic but found another called Premo Probiotic ( on Amazon) that is similar that works well for much less.

You don't say how much metamucil he is getting. It may not be enough. My nursing moms get a rounded tbs of metamucil twice a day, with meals. Of course, they are eating much more than 2-1/2 cups per day. Probably more like 8 cups per day. Since doing that I never have the common stool problems any more. It can't hurt to increase and see how he does.