Hi scoobie. I'm glad you were able to help the little guy but unfortunately, he sounds like the conditions he was kept in prior have taken a definite toll on him. From your descriptions of his conditions and of his symptoms before you rescued him, he most likely has extremely severe ammonia poisoning and that has probably pretty much killed any appetite. The ammonia poisoning has probably also led to a secondary bacteria infection and what sounds like possible ich as well. :( Poor guy.
You've been getting some very good advice from everyone so there's not too much I can add. If it looks like he is having some trouble rising to the surface, you may want to lower the water level.
In regards to the floating - if the temperature of the replacement water wasn't similar to the water you took out, this can cause a temporary buoyancy problem that usually corrects itself as the temps equalize.
I would also suggest switching to epsom salt, 1 tsp per gallon. Magnesium sulfate from the pharmaceutical section of most stores. No added dyes or scents. This will help with his internal problems, which at this moment I think are more serious than any possible ich.