That looks serious. It needs to be medicated before it gets into his bloodstream and to his organs. IF, it is actually tail rot. Don't take your chances though and have a vet take a scrape of the area to test for bacteria. The other thing this can be is bad circulation in the tail causing the tissue to die, which is actually usually the cause of tail rot in the first place. Have a reptile specialist look at it, and then soak the entire tailtip in betadine (watered down to look like weak tea) twice a day along with any medication they may give you. Tail rot can reach a point where it becomes highly aggressive and can move up the tail at a very fast rate. One of my beardies had this and just recently recovered from it, minus about an inch of her tail!