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.