I've been thinking about this and cant help but wonder how this fish lived this long without a filter. I'm thinking that there is an undergravel filter in the tank and that's what the air bubbler is about. Take a picture of the setup if you can. If so, that is enough of a filter as long as you don't remove the rocks and wash them which it sounds like you may have done after rereading the posts. If that is the case, then it will eventually recover but will need allot (daily) of water changes while waiting out the cycle.
If you already have an undergravel filter in place, you can probably take back the extra filter you just bought and spend your money on something like a couple of buckets and maybe an extra heater to preheat the water in the bucket so that you can do up to a 100% water change easily.
I can't stress enough how desperately your fish needs new water with no ammonia in it. It will be the best medicine that you can give him right now. In fact, I'd just try to get a bucket of water that is the same temp as the tank water and just put him in it with an airstone while you work on draining your tank and putting new water in it. Right now if you can. 10ppm of ammonia is extremely high.
To help determine if you have an undergravel filter, is there a plastic frame under your gravel? If so, does it have a clear plastic tube coming up from it with the bubble stone inside there? If so then that is an undergravel filter system. Your gravel is the filter media that the bacteria grows on and the bubbles aerate the water and also cause it to flow like a weak water pump down thru the gravel and up the plastic tube.
Youtube videos will show you how to vacuum your gravel and such while siphoning water out.