Can you tell us more? For instance:
How old is the fish/how long have you had him? Have you noticed any changes in the fish's appearance (coloration, bloating, fin loss, rapid breathing etc)? Has the fish been pooping? If so--does the poo look normal (short brown pieces)? What have you been feeding him? Do you know your water parameters (Ammonia, Nitrite, Nitrate)?
The answers to these questions will help us help you. Generally though, my reaction to your first description would be to increase the water changes--I would also move the fish into a larger temporary container, such as a plastic rubbermaid/sterilite storage bin. You can get 4 gallon ones from walmart or target for $3, you want to get one that is longer/wider than it is deep--this means that you can lower the water level and still give your fish plenty of swimming space--this is ideal since your fish is tiring himself out every time he needs to go to the surface to breathe. You may also want to do this in case you need to use medication--it's always preferred to use medication outside the fish's normal container. Some meds contain dyes that can stain decor or surfaces in the tank, among other reasons.
It may also be a good idea to add an airstone to the tank, but make sure that you tie knots in the tubing or use a gangvalve to decrease the airflow.