[Maybe NSFW] Help with my dog's overactive "backside" glands?

(Not sure if anal glands are a “SFW” topic!)

My dog’s got what I guess are overactive anal glands, although they may just not be expressing properly when he takes a squat? I’m not really sure.

Had them expressed by the vet just a little over two weeks ago, and within the two weeks he was leaking again. It’s been worse the past few days. He’ll just be sitting next to me, and then I’ll smell it and he’ll start worrying over it—licking the spot it left on whatever he was sitting on (thankfully it’s usually a blanket I can throw in the wash), and also licking his backside.

The vet said that anal glands can bother a dog when they’re impacted, and that often it’s a result of an allergic reaction to food—they also said that the anal gland fluid will usually be thicker or gritty… But when they expressed my dog two weeks ago, they said everything back there seemed healthy. (And only one sac was full, but he had leaked the night before the checkup.) So I was left with the impression that it’s not an allergy issue.

He also only seems to fuss with his butt after he’s leaked. It’s more like he’s trying to clean back there than itch anything, and things I’ve read online seem to indicate that when there are issues with the anal glands, there is also usually itching back there? So that confuses me, too.

If you’ve gotten this far, thank you! I know it’s not the most appealing of subjects. I’m just wondering if you might have any ideas that I haven’t thought of—on what might be causing it, or how to help it?

The vet also mentioned that raising the fiber in his diet might help. They suggested adding some pumpkin puree to his food. I haven’t done that, but I was wondering about finding a dog food with a higher fiber content. What should I look for in the ingredients? And/or what should I avoid? Or is adding that puree my best bet?

I have been feeding him grain-free food most of the time. Would that have an effect? I thought it was healthier, but I don’t really know, and I’ve gotten a lot of mixed information about what’s the best diet for dogs.

Thank you!