That does look like tail rot to me...
You can try
bathing in warm water mixed with Beatdine (diluted to look like weak tea) If he normall drinks in the
bath then wait to add the betsdine after he's had his drink.
dry the area with a clean towl and apply polysporin.
Make sure he is not kept on a loose substrate such as sand or walnut shells, if so switch to paper towel for the time being and monitor the tail religiously, if it continues up the tail you will want to get to a vet asap before too much of the tail becomes necrotic and is lost.