It's interesting that the areas are just around the edge of the shell and nothing in the seam...The carapace looks OK? How abrasive was the surface with the slanted side? Was he moving around a lot in the container trying to get out?
And what do you mean when you say you went to change the water because it (the container?) was nearly empty? Are you keeping your turtle in really shallow water? He's being kept in one of those plastic lagoons, is he?
I looked at some of your previous posts. You said you were "upgrading" to a Rubbermaid container that's 6" X 11.75" X 4.5". This is as good as a deathbowl (lagoon). You cannot expect a turtle to be healthy in a container like that...And members previously have suggested ways to improve your turtle's health and asked you for more info, which you never gave...
I'd ask more about your set-up, but I have a pretty good idea of what it's like. You need to improve it fast.
You said he's been sleeping a lot the past few days...This sounds like something is really be wrong... Take your turtle to a qualified herp vet and do some reading on what kind of habitat and diet he needs (read your previous posts and responses if nothing else).
Where exactly are you located?
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-