ttubbs, It might be the brain eating dieses Basement Bettas was talking about, it might be swim bladder disorder, it might be an internal or external parasite, it could simply be age, insufficient food, or a result of very poor water conditions. There isn't much to go on. When you find a sick betta it's really easy to get sucked in, but it will take work, time and money to save any sick betta. And it doesn't matter if the fish has ich or something more serious, you have to be very careful not to spread it to your other fish. Decide if you can keep the fish in quarantine, in a heated, clean tank. Can you afford caring for the sick fish? And are you willing to accept a dead betta if your treatment doesn't work? If you can convince the store to give you the fish at a discount or for free, I would do it, but from the sound of the fish, he/she may be close to death and you might be unable to heal him/her.
We recently lost one of my daughter's mollies to old age. He was 2 years old when we go him and he lived 2 years. That's a good life for a molly, but at the end he stopped eating, got very thin and started swimming in circles at the bottom of his quarantine tank (I didn't want the other fish taking his food, as he was having such a problem getting to it before them.) From your explanation, he sounds like an old fish who's on his last days, but that seems unlikely, and the fact that he's laying on his side at all makes me think he's too far gone, but that's just my opinion.
I'm curious to know which way you went with this. (hate not having a computer.. can't check forums very often)
Oh, Basement Bettas, I'm curious what illness you're talking about. It's not one I'm familiar with.