Before jumping to Tetracycline you should try using salt at a full dose and increasing water changes. Treat with 1 tsp/gal Aquarium Salt and perform daily 100% water changes. As OFL lady said, it will be much easier to premix the salt in a 1 gal jug of dechlorinated water. QT him in a small plastic container (the one he came in works) and float it in his tank so it stays warm. Use an extra container to warm clean premixed water by floating it in the tank as well. If you use your Stress Coat as the dechlorinator that will aid his fin repair.
Since you haven't been using a full dose of Salt it should be safe to use it for an additional 10 days at the full dose of 1tsp/gal. Stop after 10 days but continue performing 100% daily water changes until you see some fin regrowth (clear webbing/tips). At that point you can free him but try to do at least two 50% water changes per week to prevent a relapse. Good luck and keep us posted :)
here's how he's looking now. I actually did end up using the Tetracycline upon recommendation from my local vet (took the cats in for check up and vaccination and he's on a second dosage. Though, we're doing 50% water changes daily rather than the recommended 25%. He's perky and didn't seem too freaked out by the meds.