At that size he can comfortable handle a small adult mouse like a 5 week old or a bit bigger even. It's better to err on the side of caution though. Try a young adult mouse and see how much of a bump in the belly it leaves behind. If the bump is barely there straight after he's eaten then move up a fraction. If you're picking a good sized mouse and your temperatures are good, the lump will be gone by 24 hours or so, perhaps a little longer if it's an exceptionally large meal but if that's the case, it's better to go down a size again.
I mix it up a bit with mine. Sometimes I'll feed a mouse that's a really good effort for the snake to get down and then I'll let it go a little longer between feeds and follow on with a few smaller mice for a few feeds just to mix it up. Once you get a good feel for what your snake works well with you might want to do something like that too. In the wild, they don't get 1 perfect sized item every 5 days!!! -it might be a few weeks or perhaps even a couple of months between meals and sometimes the opportunity to stuff itself full with a nest of baby birds comes up.
Corns are very adaptable and that in part is one of the reasons they're so popular as pets....besides the rainbow of morphs to keep tempting us to get 'just one more'!!!