I have an adult female that I took on that I am dieting as well. One thing I was told on another forum for fatty-fat-fat colubrids was LOTS of handling... let him out to roam, run through your hands, etc.
My girl has major middle aged spread on the lower body... fat deposits at the end of the tail. I thought there was actually something wrong with her, as I had never seen this before.
And she is feeding A LOT less now than she was. Not sure an overweight 8 year old corn needs to be feeding every 7 days on a too big rodent.
Good luck with him.