I hope your little guy pulls through. I wish I had advice for you.
Are all IP meds rough on a fish? Are there some he could try that would be easier for him to take just in case or is it better to get him stronger with snails first?
Agree on the 2-3 gallon tank for snails idea, and you could do shrimp in there also.
I've also read that you could do snails in something like a tupperware container or bowl, etc. They don't really need much to reproduce other than food. Not even a heater or filter if you change the water and don't overfeed.