I wouldn't heat the mouse up in the microwave. That could've been the problem. They're very picky about their prey. Some won't eat cuz it's too cold, some won't eat cuz it's too hot. Heating the mouse up in the microwave runs the chance of the mouse blowing up, and you having mouse insides to clean up.
If the person you got the ball from was feeding it live, I would feed him a few live meals and then try to make the switch to f/t.
Your warm side needs to be in the 90-92 range. Then cold side about 85. Ambient temps need to be about 80. Also the biggest factor is a nice tight fitting hide.
Edit: Oops, missed the size of the tank. If he's still small, then a 20g is too big. Put him in a 16 oz tub or a ten gallon tank and that'll make him feel more secure.