your clown really isnt looking well.
to be honest just about any treatment right now will probably kill the clown , i would feed the crap out of him and then decide to qt, but be prepared to go 50/50 on whether he makes it or not , as mentioned he is in bad shape and looking the way he does his days are numbered.
he needs very little stress so make sure he isnt being picked on and is eating well, if he wont eat like krakken mentioned start on the garlic.
this is a very good reason why qt has to happen to all fish , if qt cant be done then you have to be very diligent on the fish you choose.
when buying clowns try to go tank bred as theres a less chance of a disease as breeders usually have that licked
most of the off the shelf meds are just that , a quick off the shelf hope lol truthfully there are only a few methods that will cure him all of which are a better preventative vs a actual cure when a fish is in rough shape as it always involves removing the fish , acclimating him to qt and then getting through treatment.
i should note that if you do want to treat , all the fish need to be done and the tank has to go fishless for under a dozen weeks.
good luck but not a whole lot of options.