Watch how much you feed him, with him being so skinny it can cause major issues. Feed him like 1 pellet 3 times a day or one blood worm three times a day, etc. If you can try to get some New Life Spectrum grow or Attinson's Betta Pro, they are both very good pellets and will help him pack on the pounds (or grams LOL) the NLS Grow is especially good. That's what I'm feeding Lee right now who was starved to death before he was rescued.
I think if he makes it through the first 24 hours he has a good chance of making it.
My opinion is to keep a close eye on him, and see how he does over the next few days. I got 3 Male Betta's from Petsmart, a really peppy Crowntail, and 2 regular ones, 1 of which looked kinda sickly. The one that was sickly or crippled looking is the only one that lived and so I got 3 others from another petstore called Animal Jungle, I have a half-moon, double-tail half-moon, and a dragonscale. The sickly one eats when I feed my betta's (I also have 10 females in a larger tank) and he'll swim around a lil bit every now and then, he always looks like he's a bit crippled when he swims but he's perked up so much after I bought him. He looks like he may be crippled permanently but doesn't show any signs of suffering so I take good care of him. Depending on the betta, you can usually tell if they want to live or if they've given up. Just keep a close eye on him for a few days and see if he perks up, if not, well, I guess euphanization... :(
awww, poor little fish. my betta, riley, (the one in the picture) was a walmart rescue. i'll have had him for six months on thursday! i didn't think he'd make it two months, as walmart bettas are not usually healthy. but he's the picture of health! i think buggle will make it! do you have a heater in his tank? turning the temp up to 85 and adding aquarium salt may help! get well soon, buggle!
He's swimming around a lot, has a big bubblenest, and flaring (but his fins are still clamped. Hmm.)
I don't need a heater here in the summer, it's really hot. And he does have aquarium salt in his tank.