It doesn't sounds like any of the common forms of synaesthesia, but there are over 80 known types and some are rare. But really, it doesn't matter what we label it, the pupil has Problem X and is under medical supervision for a number of things including X. We don't need to re-diagnose
The only time the name might actually matter is if AB want a 'label' rather than a description of the problem, when it comes to assessing how they can meet his needs - but I'd hope that 'what the problem is [and how can we work around it]' will be of more importance than 'what's the problem called'.
Thanks, Maizie, for your very practical post. Unfortunately, it doesn't list synaesthesia under the special needs section of AB's website, so - if I can get a clearer picture of how it affects him, and a letter from a suitable person - I will have to contact them direct. Thank you to all for the feedback.