Hi James,
Welcome to Fishlore.
I'm sorry your guppy is ill. I would put him in quarantine in case it's contagious and to keep anyone else from picking on him.
Since you just bought them, my guess he was already sick.
It's always good to quarantine new fish for 2 weeks before adding them to your tank. I made that mistake once, and lost 2 fish.
20 snails seem like a lot, although I have no experience with them, but I have read they are big waste producers.
Are you testing your water? If so, what are your water parameters?
Here is a link that might help you figure out what is wrong.
http://www.fishlore.com/Disease_Main.htm
Good luck, I hope he makes it.
Oh, I forgot, I would also do a
water change, when was the last time you changed it?