Boy, I don't know. Hard to tell from the picture, but it doesn't look like shell rot to me. It looks like bone. I'd have to see it in person, so it's a good thing you're going to the vet with him.
Could you get a better close up picture, like really zooming on this area? What does it feel like? Is it dry? Soft? Crumbly, flaky? Or hard? Glossy? I've seen pics of RT's labeled as "shell rot" that looked similar but those had some serious comorbid conditions so that was a lot more complex than simple as "fungal infection" where we treat with with athlete's foot cream. Those animals had been severely malnourished, vitamin&nutrients deficient. Is yours otherwise healthy? With optimal environment and great diverse diet?
I does look like the keratin layer (scute) has been chipped off. You can see the underlying bone and bone seams.
Is there something where the tortoise normally sleeps/hides that has a hard sharper edge that hits that part of the plastron. It doesn't look worn, but chipped as the edges of the damaged scutes are jagged.