Interestingly, he had the problem again (even days after I cleared out the compost). And, his brother, Sonny had a minor problem with it just recently, too. I gave them both the Metamucil wafers again (in small pieces), 1/2 for Rocco and a much smaller portion for Sonny since he hadn't had them before. BTW: They LOVE them! An easy "sell". I think it worked again, because several hours later they were pouncing on each other (and everything else), again. They eat and drink lots of water and otherwise seem fine. I'm going to abandon IAMS and gradually switch to Nutro Natural Choice (the Lamb/Rice version). It doesn't contain any other grain. If that doesn't work, I'll try a grain/gluten free food. The other possibility that could have caused it are the treats I give them occasionally (all natural, but have oatmeal, whole ground oats, ground barley and Rye, along with natural cheese" (way down the ingredients list).
I won't take them to my vet until it's time for their next round of shots (another 2 weeks, I think) . . . unless the situation gets worse. When I do, I'll let you know what she says. I really trust this vet -- she always gave me good advice when I had my other three dogs.
BTW: I'm not a health nut, but I AM health conscience. For the past 5 months, I've voluntarily gone "gluten-free" as much as possible and can feel a big difference in my joints. I can't help believe that other carnivorous animals could easily have issues with grains, too.