@ajrwdwgirl ,
@FutureCEO ,
@Wrangler-Rick ,
@figmentfan423 , and
@catmom46 , thank you all for your input about dog food. I went out and bought 1 small bag of Nature's Recipe, grain-free formula with salmon. I'll try a little of that tonight for him, and through the week to see what happens. A vet had recommended that brand, so I'll give it a try. (It does cost about 3 times as much as my previous brand.) Something interesting: I noticed that all the dogs here are eating different brands. Again, it just comes down to their personal preference.
I was stunned to read from Wrangler Rick about how sometimes companies change certain ingredients based upon price and/or supply, but aren't necessarily forthcoming about some of these subtle adjustments. Apparently, catmom46 was also aware of this issue.
In conclusion, I suspect this may be a period of trial and error to see what works best. (We had just taken him to the vet last week for a complete physical, vaccines, etc., and the vet didn't see anything wrong at that point.)