1. I know how big that tank is and I KNOW that it's not big enough for a Betta. Period.
2. You don't need to spam the boards with a new thread each time you make an update. Just reply to your first thread you've posted. That's how I've been able to make updates with my Tian-Tian's condition. I'm not trying to troll on you; it's just a simple fact. Overloading the boards does not help.
3. I suspect swim bladder or dropsy. Swim bladder is relatively fixable, given some time, patience and love. If it's dropsy, however, there's not much else you can do. Are his scales sticking out away from his body (kinda like a pinecone)? It looks like it, given the photos. If so, that's dropsy and few Bettas bounce back from that.
I'm sorry I'm not of much help or comfort.
Let us know how things go.