Since it seems to recur once he's been on kibble for a few weeks, I'd have to wonder about an intolerance to something in the food. If the intolerance is mild, it would take a while for it to make him sick.
I would also suggest trying the Orijen Senior - it's grain free, so if the symptoms are caused from IBD or something similar, you should see some improvement. Also, make sure that you aren't over-feeding. IMO the feeding guidelines on high quality kibble are overstated.
Bless you for adopting this guy! The elderly ones are always hard to place.
Livin in a Newfie Drool Zone