Welcome to the board. There are a couple reason why certain cats lick that area - the first is fleas - even though kitty is an indoor cat, he can still get fleas that we bring in on our clothes. Another reason is there are glands near the base of the tail - there is one on top that causes "stud tail" even in neutered cats and there are also anal glands on either side of the anus just below the base of the tail.
If it continues even after you've applied flea medication, I would get it checked out to make sure there are no infections in any of the glands.
Good luck!
Cat =^..^=