Glad you posted that, it sounds like you got bad info on how to care for him when you got him. I know it can be tempting to think you've done it one way for so long with no problems, but when they start bloating like that it means things have reached a critical point and they will keep getting sicker and eventually die unless things are changed.

The tank is too small; assuming he's a male (you can check for nuptial pads on the back of his front feet if you're curious; yes means male, no means female) then he'll need about a ten gallon. This is important in part because when they pee they pee a LOT; having the larger tank allows them to move both to get away from their own waste as well as to regulate their temperature.

Have you started treating the water? If you don't this will keep happening and he'll die fairly soon. It's probably the most important thing. You'll need to change the substrate for some that's expanded with treated water, as well as the water dish.

The feeding is wrong as well. You should never feed him a mouse older than a pinkie, and not more than once a month as a treat. The fat content in mice is way too high and can cause liver damage and blindness. Also no need to dust mice; they have calcium in the bones.
Crickets, nightcrawlers (from WalMart works fine), and dubia roaches all three make excellent staple foods. It may take a couple weeks to get him to start taking those well, but it'll be worth it. Those are by far healthier foods.

Also, temp needs to be 80-85 daytime and 75-80 nighttime. Lower temps will eventually cause lymph failure. Humidity should stay around 80%; dropping far below that will cause them to start hibernating. You can buy an infrared thermometer for less than 20 dollars at most chain petstores-- those are the most accurate.

I hope that info helps get him on the right track and that he stops bloating up! These frogs can live 10-20 years if taken care of properly, so making the changes will be worth it.