It sounds like he has an infected wound or maybe an abscess. Treat with a strong anti-bacterial medication such as Esha 2000 or Waterlife Myxazin. Paraguard won't help at all as it's a treatment for external parasites such as gill flukes
Thank you so much for your advice, I’ve ordered esha 2000 and will let you know how it goes. Does the medication it have any effects on the other fish in the tank? Should I be separating the fish with the problem into a smaller tank?