I would call that shell rot. When I rescued one of my turtles from an aquaintance who took notoriously poor care of him, he had similar shell issues...not the coloration that yours had, but similar pitting. The most important thing for his road to recovery is that you have him in the proper home that he needed and deserved. So, that part is already done! Proper heat, and basking work wonders. Just remember though that shell issues like that are a long term recovery process. I have had him for two years now, and much of his pitting has healed with the new shell growth from underneath. But there is still a bit left that will take more time yet to heal.
There are several opinions out there regarding possible treatment of shell rot. People here on the forum have advocated the following options: 1. Go natural, not use any kind of topical agent. Let it heal with proper living conditions and normal basking. 2. Use a topical Betadine solution a few times a day followed by a few hours of dry docking each time to disinfect the shell and keep it clean 3. Go almost complete dry dock and only provide a few hours in water per day so they don't dehydrate.
My recommendation is that you do not use option 3...given the extent of the pitting he would be dry docked for a long, long time and that just doesn't sound reasonable. The most important thing is that he can now bask and get dry, and that his water stays very clean. Do some reading up on different ways to treat shell rot and decide which will be best for you and your turtle. Good for you for helping this turtle and best of luck!