Yes, some great advice. Corns are known for being prone to getting overweight. You can easily tell by looks, and feel. They will have a "squishy" feel to them.
A couple of my male corns started to get a little on the pudgy side. What I did is to make sure I handled them a lot. Maybe 15-20 minutes 5 days a week. I also decreased feeding from every 7 days to every 14 days and went down in prey size from x-large mice to just large mice. They'll be a lot more active, but don't let them fool you, they're getting plenty.
After a few months mine had gotten that firm feel again. I don't really keep up with the handling part as often, but I still do feed every 14 days and only lg mice. I've since switched all my adult males to only 14 days to keep them trim, and its working very well.
I wouldn't cool him or change temps to get him to lose weight. That will just make his metabolism slow and he won't burn anything so won't lose weight.
Keep in mind that he won't be back to normal overnight, it took a while for him to get big and will take a while for him to lose it. But after a few months of decreased feeding and increased handling, you should easily get him back to being slender and in shape.
Good luck!