How long have you had him? (edit, sorry, I see you've had him 5 days)
He could be cold. You may need to get a heater or warm him up with a space heater and towels.
Do you remove the uneaten food within 5 minutes? What brand food?
Is he pooping? Sounds odd but if he's constipated he won't really want to eat.
If you can upgrade to at least a two or three gallon, that would be best but I understand not everyone can afford or deal with that. Petco is having their $1 a gallon sale at most locations so you can call around and at least have the tank and get bit by bit as needed.
Are his fins unclamped?