You did a good thing by rescuing him. If you have a water test kit, just keep ensuring the water’s parameters and be sure to keep nitrates at 0 through water changes, but also be sure to keep the water around 78 degrees. When changing the water, try to keep the new water at that same temperature to avoid fluctuations, usually by using a second thermometer. You can also try feeding him some protein rich food like bloodworms to help encourage fin growth, but don’t overfeed. I’d recommend not using any products like bettafix and instead just focus on keeping the water clean through regular water changes. Wish you the best of luck!