New Enclosure feeding problems

I have a male Pied ball python. I got him in April of last year at about 150 grams. He is now about 3 foot long and 850 grams. About 4 months ago I moved him from a 20 long enclosure to a 40 breeder. Before the move he never missed a feeding. Since I moved him he has only eaten 3 times. All of my numbers are consistent and checked with temp gun regularly. His enclosure is very basic with little decoration. It has 2 small suction cup plants, a hide on the cool side a hide on the warm side and a water bowl in the middle. Could it just be he hit a plateau in weight and isn't eating or could the bare cage be a problem?

150 grams is not a lot... And because you said that the enclosure is very "basic", the problem probably lies in the snake feeling exposed in such a large tank. Clutter the space with fake plants, add more hides. Make sure the hides are tight and touching his sides and use the same type if you can. Add extra layer of substrate for burrowing.

Also, please list your temps. And do you use a thermostat?

He was 150 grams when I purchased him at a local expo. He is now 850. My high side is 90-91 and my low side is between 79 and 81 consistently. My humidity ranges from 50 to 60 depending on if I spray that day or not. I am using a herpstat ez2 to control an under tank heater and a CHE. I have 2 identical words terra reptile hides that keep him tight in the hide.

The solution is really in the title of your thread. Move him back and see if he will eat for you more consistentatly in his old enclosure, if he does than he is not ready for the bigger enclosure, if he doesn't than he reached a plateau either the wall or being overfed and catching up.