Oh yeah, arsenic! Yum!
I would be looking at intake and output like you are already are. So I would be feeding those easy to digest food combos, and probably adding a little salmon oil (probably more to make me feel better than it doing anything). I would be watching very closely for any vomiting, any lack of interest in food or treats, any change in behavior, poop changes for the worse either hard or soft, and lack of interest in water. If any of that, vet call immediately. I would keep the vet updated anyway.
I would also bring a fecal sample in to check for whips, etc. I drop random poop and pee off, not sure if I am "spoiled" or if it's common to be able to do that without the dog - if the dog has been seen in the past year, my vet office is fine with it. But this would be a must-do.
I think pumpkin would be a great thing for you - it helps to balance the poop between soft and hard, which is really important. My GSD actually had a prolapsed rectum (oh yes - thankfully only a bit and the vet had to do nothing) this summer from pushing too hard - I had gotten her poop firm, and then...too far! So now she gets a considerable amount of pumpkin.
She gets a liquid probiotic - just Acidophillus - human from the grocery, too. That might help him a bit.
Hopefully someone else chimes in - I hate to be the only point of view.