I never have any luck with guppies either. I keep all kinds of different fish, and the guppies never seem to last. Although I do have about 40 fry from 2 of my females, and they seem to be doing well.
You can just get a bad batch from the store- i know i have in the past; if your guppy is actually suffering from a desease i wouldn't buy anymore until either he's got over it, or in a worser scenario he dies- if he does die i would leave buying anymore for a week or 2 at least.
Its posible your guppys had in internal bacterial infection- the symptoms are quite hard to tell usually but usually you get alot of sudden unexplained deaths when theres nothing wrong with the water quality and the fish has white stringy poop.
If you want to still have guppys i would try buying them from a different store just in case you did happen to get a bad batch.
Okay, thanks Tokis. Like you said, I don't want to add new fish 'til he's recovered, but I also don't want him to be lonely. I've put a hand mirror next to the tank glass and he seems to be swimming around more... he probably thinks it's another gup. Poor little guy...
My dad wants me to get him a couple of females, as everyone here's said their fry have been stronger..