Any idea why my Labrador smells like a wet dog, but only on his back?

Wilson has smelled good all his life. We moved in July, and since August there’s been a persistent problem with that wet-dog smell. Not always strong, but always perceptible if you sniff him. It’s located only on his back, not around his neck, but closer to the tail…what would be the lower back in a human. It’s a distinct “wet-dog smell”. I guess he only smells bad where his fur is thickest, but I’ve brushed him, and not much fur is loose. He’s in his summer coat and not shedding much yet.

At first I thought it had to do with our lake/pond behind the house, but the smell is not related to swimming times, and keeping him away from the pond does not improve it. In fact, he reliably smells good on the day after a swim, and then gets worse.

Then I figured he must have rolled in something (which is unlike him), so I shampooed him…he smelled great…for two days. There is no way for him to have accessed any ponds or rolling opportunities in those days. And yet, the smell came back. We don’t have any skunks here. He’s not itchy or otherwise showing signs of discomfort. He gets a high-quality dog food that’s “human-grade”. The other dog, female, smells fine.

I’m out of ideas at this point. To clarify: I’m not looking for tips on how to deodorize him, the dog shampoo is working fine. But the smell comes back within days, and I really don’t want to start bathing him that frequently. I can’t imagine that would be healthy for his skin, either. Most of his life, a quarterly bath was plenty.

What is going on? Is it time for a vet visit?