Swollen but not pineconing, definitely lethargic. Hmm. The swelling has steadily taken place over the course of a few weeks? Sounds like either a tumor or a severe internal bacterial infection. In a case like this, I'd definitely take him out and put him in a small "hospital" tank (if you haven't already). Something that is approximately a gallon will work best for treatment. Start him off on 3 tsps of epsom salt per gallon. Change the water every day and redose the salts (every other day if you're really busy). At this point, your best bet in the medication department would be Seachem Kanaplex but it's a hard medication to find locally unless you happen to live by a really good tropical fish store. It almost always has to be ordered online. Try www.kensfish.com
My main worry is that it will be too late by the time it arrives. :/ Your other best option would be API Furan-2.
If you can, the addition of tannins would be helpful. You can get these using Indian Almond Leaves (IAL) or by crumbling naturally dried, clean oak leaves into the water until it turns a tea color.
When you treated him with the tetracycline, did you treat him in your planted tank? If so, did you remove any carbon from your filter? Carbon will adsorb all medications in the water, since that's basically what it's designed to do. Also, I'm not sure but the tetracycline could cause your good bacteria in your filter to crash a little. Some meds say they're filter safe but they really aren't. When treating with meds, it's always best to do so in a hospital tank if you can.
I hope the poor little guy makes it. Get well soon, little betta buddy! Good luck and keep us updated.