I would just use aquarium salt and change the water 100% everyday for a max of 10 days. Only changing it 50% when using salt can actually be unhealthy when you add additional salt in with the new water. If you can move him into a smaller quarantine tank so that the 100% water changes are easier. Doing that with a 5 gallon would be a pain!
I'd just do 1 tsp aq salt for 7-10 days, making daily 100% water changes. Keep the water temperature around 77, cooler temperatures help inhibit bacterial growth. Aq salt treatments work great, so I'd stay away from any potentially harmful medications/chemicals.