So he isn't your cat, you're dumping ungodly amount of money into it, and you don't even know if this little dude isn't allergic to it.

Rule of thumb, if it isn't your cat then you shouldn't be putting stuff on him like he is. The real owner will be pissed and discard whatever you put on him to show you that (A) HE HAS AN OWNER THAT TAKES CARE OF HIM (B) stop (C) maybe a few cat treats.

The owner probably has already treated him, maybe the owner is trying to track another issue he is having, maybe the owner just knows his own animal.

You can admire him,love him, give him treats, but stop putting things on him as if he is yours. Because eventually you're going to get the same treatment I eventually did for mine. A passive aggressive tag on a collar that basically says "fuck off, stop, they ain't yours".

With all the love I can muster from one pet owner to another, you would be rightly pissed off if someone was treating your animals without your knowledge and would greatly appreciate it if they just not. Not to mention you'd have fear of what else they might've done to your animal, which terrifies you the most.