First start with more frequent water changes, with no filter or live plants you should be doing about at least a full water change once or twice a month (if you have live plants, you won't want to do a 100% change), with a minimum of 1 50% a week. But if he's sick, you should bump it up to at least 3 a week.
There are mixed feelings about bettafix here, with the general consensus being that it's actually dangerous because it contains a type of tea tree oil that can damage the labyrinth lung. So to be on the safe side, do a complete water change, and don't add anymore bettafix (there was a case recently where as soon as the person quit using the bettafix, the fish got better and more active.. Do you have a hospital tank you can put him in to make water changes easier? You can always float a large tupperware in your tank to put him in so he can reach the surface easier.
Another thought is that he's been biting his tail, which would cause it to appear not to ever get any better. In that case, you don't need any medications at all (although aquarium salt won't hurt as long as you're using the recommended dose for a maximum of 14 days). The lethargy in this case could be from over medicating. If it turns out to be tailbiting, you'll need to find ways to keep him entertained to break this very difficult habit.