A corn snake that size could eat an adult mouse once in three weeks. Corn snakes get too fat very easily, so one in every three weeks would probably be more suitable.
And, I wouldn't recommend feeding him in his terrarium. The most corn snake owners have a separate feeding box. It can be as simple as a plastic storage box with air holes in the lid. Corn snakes may accidentally swallow some of the bottom material if they eat a wet mouse in their terrarium, in which can cause a painful death due to bowel obstruction.
If you don't feel conforttable moving it to a separate feeding box (just for the feeding), then you could also use a some kind of plate or something like that what you would put the mouse on.
I myself have three corn snakes, a 1,5-year-old, which eats every 10 to 14 days, a 4-year-old which eats every 14 to 18 days and a 6-year-old whish eats every 3 to 4 weeks. They eat in large buckets.