You should consider him a very lucky tortoise. It could have been much worse. The white is exposed bone. That bone will die and in the meantime, new keratin and bone will be growing UNDER the white, dead bone. This takes a very long time. After a couple of years or more, you will notice that the white seems to be raising up. It will eventually flake off, exposing the new keratin underneath.
I took this Russian tortoise in as a rescue last summer. He had previously been dog chewed, and the most of the old bone had already flaked off. You can still see some of the 'white' on the front edge right behind the head. The new keratin under the flaked off bone is bumpy, but I think it will smooth out:
That's pretty minor. The tortoise in the picture I posted for you had the white exposed bone showing all over the whole carapace. Your tortoise is going to be fine.