Since a few days ago my dog started licking at his butt and dragging it along the floor several times a day. What's wrong with him?

It can be an allergy, an infection or abscess in his anal sacs, or they can be full. You need to see a vet. If itching becomes too frustrating for your dog, you can apply hydrocortisone cream (2%) or warm compress on the area to relieve his condition before you're waiting for a vet appointment.

Thnx! Can this be because of worms? And what can be done if the problem is full sacs?
I would not apply any hydrocortisone cream that can be toxic if licked. Call the vet asap! If the anal sacs are full the vet expresses them there at the office. I've watched it with my sister's cat, it's not painful at all and doesn't cost much. It needs to be done. If it was worms he'd have diarrhea and/or blood.

If a small amount of hydrocortisone cream is ingested, it doesn't usually cause a problem. Large amounts can cause stomach upset, increased thirst and urination. My advice to apply it was meant as a temporary solution to relieve the dog's condition while waiting for a vet appointment. I did not mean applying it over and over again until the dog ends up licking off large amounts.

I understand but I still don't recommend it. You don't know how much can do any harm and I don't like applying ANYTHING that can be toxic, even a little bit. Her call but I don't like the idea at all.
Growths, swelling, or irritation: These are fairly common in the region around a dog's anus. ... You can treat irritations at home with a topical antibiotic ointment, such as Bacitracin, or a steroid, such as hydrocortisone. If they don't work after a few days, call your dog's vet.
I took my dog and had her glands done but her butt is still itchy
I already saw the vet they cleaned Her anal sacs