Before he reaches adult size (at around five feet and three years old), he should eat food that's about one and a half times the width of his body (not his head or neck). It should leave a visible lump for 48 hours after eating. When it no longer does that, it's time to move up to the next size. Once you reach adult mice, that's the best size for an adult Corn and he'll eat those for the rest of his life.
He's pretty much fully grown at four feet, so I'd try to increase his food size. Try him with a small mouse and if he manages that OK, give him those for the next few feeds. Keep an eye on that post-feeding lump in his belly and the minute it doesn't last for 48 hours, move up to the next size of mouse.
Beware that he will take longer to digest as the food size gets larger, so leave it longer between each feed. With small mice I feed one, once a week; medium mice, one, once every ten days; and large mice, one, once a fortnight.