Aw poor little dude! But you know what? We're in the same boat! I just inherited a betta who is in almost the same condition, rescued from what I am certain was betta hell!
Sometimes it takes days to see any improvement, but it's not impossible. So long as he's warm, free from stress, and still showing signs of hope (eating the bloodworm is always good!), there's hope! I'm sending him positive vibes. Keep us up to date!