Just because that worked for you does not mean it will work for all ball pythons. I've had many balls that won't eat in larger enclosures despite how much I filled the cage up. The ambient air temperature should be 80 or above, 75 is too cold.
Get him a 15 gallon at that size. Don't use the microwave at all, just use a blow dryer if you want to warm him up after thawing him out. Putting it in hot water to thaw is a lot faster than just leaving it out.
Make sure you have a good ambient air temperature of about 80F and a warm side at 90-95F and the cool side at 80-85F. What are you using to heat up the tank? If it's a heat lamp, get rid of it and get an UTH and Thermostat combo.
How are you measuring your heat? What kind of thermometers? What are you using to measure humidity? (thermometers and hygrometers that are dial are extremely inaccurate and a waste of money, get some digital ones from the planting section in Walmart for 10 bucks. Get your hands on some digital thermometers with probes too to put UNDER the substrate on the glass to make sure it's not getting too hot).
What kind of substrate are you using?
For the feeding, turn off all lights, get an appropriately sized LIVE rodent, put it in the cage and leave him in quiet. Watching him with all the lights on in the room or any sort of traffic could be stressing him out. Also, do not move him to another enclosure if you are attempting to do so. His own enclosure is fine for him to eat in.
P.S. For appropriately sized, go with a rodent that is about as thick as the snake is at the thickest point on it's body. You could also do the 10-15% of it's body weight rule.