An excellent question as a teacher of final year International Baccalaureate ( a bit different from your Upper Sixths), I would love to know the answer myself. But as a self-identified aphant, I would encourage the following
a) as for all students, remind this student that exploring his own learning style is worth the time. While so many students now fall for the “I am a visual learner” trap, he should make a very serious attempt to note where/how he learns best in each subject. For example, I learned despite all research indicating that this is not a useful study technique for most students, I excelled in Biology by re-writing notes.
Of course, all of this has side-stepped your query about “training out there or books/articles you can recommend”.
b) Counterintuitively, making diagrams has always worked for me in all subjects. Perhaps the act of making the diagrams for someone who cannot for images is a struggle, and the struggle itself resulted in longer-term learning. So I diagram out everything (including my own lesson plans today) even though I do not think in diagrams.
c) I think this is the really tough bit …I never found it hindered academic progress in any subject, but it has affected how i make plans (He may need to plan every step rather than focusing on the final goal). So in other words, allow him to build on details.
Finally (unrelated to your question, but may be relevant in other domains as he goes into higher education), this has affected how I respond to people/ groups (finding it hard to visualize their emotions and reactions). Oddly, this makes people think I work well with others, because I end up not expressing my own emotions because I cannot imagine how they will respond. But helping him deal with visualizing emotions may be the greatest help you can give him