My little guy has been chewing on his tail :/

Does anyone know what can cause this? he started doing it like a year ago and it was always pretty small however this past month or soo it’s been getting real bad. At first we thought fleas or something but he has gotten 4 flea baths and we never found any fleas on him at all soo now we are kinda unsure on what we can do to help him out.. and my parents won’t take him to the vet :/