My DD was given the RIAS, and her verbal/visual splits were pretty similar to what you've shown here. And though I've never been formally tested, I have informal overall IQ test results that match hers, and her brain so far seems to work just like mine. We seem to understand each other VERY well.
So, based on my own personal experience, I'd say the best way to improve a V/S learner's verbal processing ability is reading. It's a visual presentation of verbal information, so it's right up his alley. The more he reads, the more easily he processes language visually. And then, when people speak, he utilizes those same processes. When he speaks, he might also find himself writing his words visually across his brain, then reading them out.
At this stage of the game, I don't even notice I'm doing it anymore, but I remember it being a very apparent process when I was in elementary school. Every once in a while I'm given a reminder, when someone tells me their name and I can't picture it.
This is just an anecdotal, unscientific suggestion from someone who may or may not work the way your son does, so feel free to disregard.
