Gout on top of a RI and mouth rot is not good.
There's not much to do to treat gout effectively and it is very painful. He may have been hanging from his tail because it hurt too much to hold onto the branch. I'm sorry to say that it does not look good. But you're doing everything you can to help him, it's not your fault! It sounds like his previous owner did him no favors at all. Just be prepared if it does not go well. 