This is not necessarily a bad thing--if the anal glands are 'going off' (so to speak) by themselves, they at least aren't going to get impacted. We had a male dog with impacted anal glands and only surgery eventually fixed the problems. Did the vet think Cooper's anal glands were inflamed or infected?
All six of our dogs have times when their anal glands 'go off' (so to speak
) periodically. I've found that dampening some paper towels and using them to wipe off under the tail goes a long way in controlling the odor. (Use more paper towel if you don't want to get any on your hands
...or use latex gloves.) If Cooper is expressing them well by himself, that may be all you need to do when you catch a whiff of 'tuna butt'.