What is causing loss of central vision in my client?

This is indeed a karmic malady that is developing. You can do work on him if you choose, that will be of benefit. He is not his best advocate, as sometimes happens with clients. As you know, that is because he is too close to the problem, but you can be instrumental in bringing assistance here. While this is karmic and has a strong energy behind it, not surprisingly, as this is what karma does, it can be countered with the healing effort. And this is what we expect will happen with some concerted energy sent on his behalf.