I had my lab at the vet this week for a recheck from her surgery. While we were there I asked the vet to check her anal glands as she was licking a lot down there and she smelt a fishy smell which I understand is the smell when they are upset. They checked them and her glands were fine.
So, why then is she still smelling fishy?? if the glands are clear??
The smell is awful. He must have residue left over from his discharging himself. I know your Vet said he was good but it sounds like he had some still there. When they express my dog, it's an awful smell but once it's emptied, it's fine.
I would have it checked again. Normally, you can tell when they need expressing. I won't get into the nasty details!
Could it be that the glands expressed on their own and there's a little residue left on your dog's back end? About once a month our dog's glands express on their own and the smell almost makes your eyes water. We very gently wash that area off and all's well again.