it could be either one. any new supplement or med should be started at a lower dose.
i have used metacam for a few years now and the dogs never had diarrhea on it, but all dogs are different. when do you give it to him, morning, night, before, or after meals etc? i would also start with half the joint supplement. sometimes chondrotin, or sea mussel can cause diarrhea.
oh, BTW. if you are giving a 100 pound dose of metacam, that is costly. you can get metacam in peoples prescription at walmart for 4.00 for a months supply, its the same thing as metacam, but its called meloxicam, your vet just has to call in the prescription.