I would say this is probably exactly the problem, since you know it's NOT his anal glands.
Well, I can only share my experience. I've had a cat who scooted several times. Each time it was because there was a little piece that was still stuck partially in & partially out. Gross, I know. So he was basically wiping himself on the floor. At the time, he was actually a bit constipated. So yes, if they have trouble with going to the bathroom & have stuff leftover, they will scoot on the floor to remove it. (once he traveled all the way from the living room to the kitchen!!) But I can't say for sure that this is what you're cat is experiencing.