That are rather a big mices... it's weird that having one every week your corn is skinny, did he has a big enclosure, or something? I use mices around 2-3inch, and my corns are growing on them really fast (but they are 2yo so they still grow rapidly), unless when one of them was sick.
Still when the snake of the OP is a little fat I would space the feeding to one adult mouse every 10-14 days, and if this would help then you just need to find how much your corn really needs. Most breeders usually suggest one adult every 10 days, so I would start there, especially that healthy corn adults should be skinny. In the wild they are rather one of the skinniest snakes, but most of the people are overfeeding them to breed them sooner, and nowadays most think that corns should be fatter. I was one of that people too, but found a great group on Facebook with some of the really nice, experienced breeders, so I'm just repeating what I learned
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