you'd proabably get more responses if you put this in the "diseases" section!
if its ammonia poisoning you'll see the gills looking inflamed and red. we really need to know your readings for ammonia,
nitrite and
nitrate, and
pH (this affects the way the ammonia affects the fish) particularly if you are still cycling.
how long have you had the fish. sometimes fish die within a week if not acclimated properly. guppies are prone to this as myself and susitna have lost guppies this way despite our best efforts to
acclimate slowly......guppies just dont appreciate their new homes sometimes. particularly the boys for some reason.
hope he makes it, and be sure to get some readings as i said earlier. in the mean time do a 50% water change everyday as that often helps with anything and everything when in doubt!!