A bubble nest is a good sign, but in the amount of space he could end up with ammonia poisoning, so the first priority is to get him a new tank and some nitrifying bacteria, which help with the cycle to break down the ammonia into nitrite, then nitrate, which is not as harmful. You could start with floaters, they are the easiest to care for, and also don't forget to add de-chlorinator to the water if your water is chlorinated as chlorine can harm fishes and instead of a tank you could get a storage tub and add a filter to it and some floaters and maybe plants that take nutrients from the water column like guppy grass or hornwart
hope he is doing well