Haven't weighed him recently. He doesn't miss a meal, but I space them out to keep him on the lean side. A corn should have a more rectangular cross section, not so round, but not sunken anywhere, either. The more healthy corn snakes you look at, the more you will develop an eye for it.
Yours in the pic looks fine to me. They grow at their own pace. While he is still small, feed once a week, a mouse about as big around as he is.