Thanks for all the input. Just to clarify a couple points:
First, I hear what you're saying about a young GSP always looking skinny. In this case, however, our vet is a serious upland hunter - like 50+ days a year - with decades of experience owning, training, treating and working gun dogs like ours. So, when he says the pup looks skinny, I pay attention.
Second, the "tell" wasn't ribs showing. It was apparently that his hips were looking a little too bony. That was a new one on me, but okay...
It's only been four full days since he got back from the vet. We added an extra half cup at each of the morning and afternoon feedings for the first couple days, and then tapered to an extra cup only once per day. Thus far, no signs of digestive intolerance. I can already notice a difference - his coat looks shinier (not that it was that dull before). I might be imagining it, but I think he looks less skinny too. At this point, I'm going to stick with the current levels for a while and monitor his weight. He's 53 lbs, so he's pretty close to full weight, but his parents were both on the big side for GSP's, so he may end up in the high 50's/low 60's.
Incidentally, I've seen some research on high protein foods for gun dogs too. The papers I read suggested that high fat (on a relative basis) is a little more important than high protein, which is one reason we switched him to Victor from Orijen. He seems to tolerate the Victor (which also has a grain component) than the Orijen (which is grain-free) too.