Hello,
Wow, that does look like something is going on with the last portion of his tail. Has he shed his tail recently? It looks like there could some constriction on the tail. Did he have his tail pinched by anything previously? Have you bathed him to see if you can get any skin off of his tail?
How long has it appeared darker then the rest of the tail?
The lower portion looks like it might shrivel up & fall off. Is the end of the tail hard?
You will need to watch for the discoloration to travel upwards on the tail, because that can cause infection. Can you get him to a vet to have him check the tail out?
Tracie