I'd agree that a physical bump can cause an eye dent, but how fast the eye plumps back up (& whether or not it ever does) still relates to the state of his hydration. Even with adequate humidity in the enclosure,
I think it's still possible that he doesn't drink enough water, especially since "he's a spider". If he eats f/t, try offering them wet. Might also try giving him a soak now & then...could be just in a container with a well-saturated towel for his stability, rather than freaking him out with the loss of traction when placed in water alone.
You might also try to encourage him to drink: gently passing a snake's face under a dribbling faucet will usually get them to drink, or if a snake is calm enough when you hold them, gently lower their face to the water bowl so the surface tension of the water gets them to drink. Many ways to do the same sort of thing...many snakes will even drink drips of water off your finger, but I can't speak for spiders, never had one. It's just that any snake with a coordination issue may not drink from a bowl when they should & when normal snakes would have on their own. With a "special needs" snake, it wouldn't hurt to encourage them to drink water at least once a week.