While it's sometimes normal to see your snake's skin between the scales immediately after feeding, that's because they swallow their food whole and it's pressing on their skin. If you're seeing the skin between the scales otherwise, then your snake is definitely overweight.
I wouldn't use the spine as a reference for whether your snake is over/under weight, as all snakes, except for the obese ones, will still show their spine. A more accurate method is too look at the shape of the snake. Despite how they're portrayed, they aren't actually round.
- A snake that's underweight will be more triangle shape, with the sides leading straight from the belly to the spine.
- A healthy snake will be rounded at the top, but flat on the bottom, kind of like a half circle. The purpose is to have as much surface area (for traction) on their stomach as possible.
- An overweight snake will be rounded, sometimes almost perfectly, with sides of the belly not able to touch the ground.
Another thing you might be able to notice are hips/rolls on you snake. All snakes will have creases when they're coiled up, but an overweight snake will have more fat in those creases and it can cause noticeable rolls, or even misshapen scales. The misshapen scales are generally the worst cases I think, they come from the pressure of the extra fat in the creases.
[Notice here too, the stomach doesn't lay flat]
As far as feeding schedules go, you should always keep a strict feeding schedule for all pets. It's especially important for corn snakes since they're opportunistic feeders, they'll eat even when they shouldn't.
Common feeding schedules for adult corn snakes are more like once every 12-14 days. That might seem like a pretty open schedule, but that all depends on what you're feeding him. Larger or fattier food needs to be fed less often. I would say start by feeding him only once every 12 days, and give him some exercise.
You can give him some exercise by having him climb up and down a set of stair if you have some, or if he doesn't hate baths, you could encourage him to swim every so often in the bathtub. One trick I really like, is giving them a box full of cardboard tubes, from paper towels and stuff. Just the new environment will encourage them to explore, and you'll have fun watching him explore the maze of tubes.
It will take quite a while to see some improvement. Just like with humans, snakes won't lose weight within a week. I know some cases where it's taken a whole year for people to get their snakes back to a healthy weight. Stick to a longer feeding schedule, and encourage your snake to exercise as much as possible, and he'll be fine.