Yay, Mermay with MerKev! :D
Poor thing has barely gotten any kindness in his life. What do you think, how long would it take for MerKev to start trusting someone and what would it take? Maybe someone leaves out fish for him regularly after witnessing him eating scraps. They aren't stupid enough to try and touch him at first (they want to keep their limbs intact thank you XD) but maybe, just maybe, MerKev would let them touch him after a while? In case he needs help with something (a splinter in his tailfin maybe that he can't pull out without help)
Hmmm... it would take a while, for sure, but it is possible. Food is definitely the right way to do it, if you're discreet enough he doesn't suspect you're trying to poison him at first.
The medical help is more sensitive, but can be step two. He can always bash the person to death if they try anything funny, after all, so he'd let them try.
Maybe it's really hot and dry outside, and one day he accepts the friendly human's offer to dip in their pool when he's all dehydrated. Days pass, trust slowly building, until they get to a point when he allows cuddles!