id be having the anal glands checked, (anal glands are little glands just inside the anus they occasionally get full or blocked or impacted and butt scooting is the common way of a dog applying some presure to try and relive them.
a vet or groomer can empty them, they can show you how too, but its STINKY!
som breeds are more prone to this than others but it can happen to ANy dog ANY age. firmer stool often heps express them naturally, but if there blocked for too long without relife they can become infected.
it could also very well be worms, just because the dogs been wormed, it also depends on what the dog was de-wormed on...and even then, unless you can 100% keep him away from anywhere where ANY other animal has been theres no saying he hasnt picked them up again. (most vets suggest a fecal test with the yearly checkup) so id suggest the same, drop off a stool sample, have them check it and have them check the anal glands too...