First off, the start of fin rot is the appearance of milky white areas on the fins. If you notice his rear fins, it's present.
Secondly, the signs of internal parasites have 2 major indicators. He's not eating at all, which will lead to weight loss, you know 1+1=2. The second indicator is his inactivity. After doing personal research (i.e. calling my local aquarium), I was told that internal parasites can also cause what we see on his tail.
Trust me, I did not pull these answers out of thin air.
Either way, you gotta do what's best for your fish, if you truly think he's sick, then treat it, if not, then don't.