Actually if you are going to treat for parasite, a Cowfish should be treated with hyposalinity. They are sensitive to copper sulphate.
I would just keep high water quality and feed him food laced with vitamins such as selcon. He should be OK.
If he starts to go downhill, then please move him to qt
. When this fish dies, it releases a toxin that can wipe out your other fish.
Keep a close eye and good luck!! Get back to us with any other questions.