Son isn't in an English-speaking education system, so there are no educational advantages for him in choosing one qualification over the other, but he is such an ears-person that he really needs to formalize what he knows. Stuff like the fact that notes are not only memorizable pitches, but that they have actual *names*, which a dedicated person might actually be able to remember for more than 24 hours at a stretch...
Any recommendations? I'm leaning towards practical musicianship, since he already thinks with his voice and his ears rather than in words. I heard his teacher telling him the order of items for the end of year orchestral concert program the other day...she didn't say "Albinoni, then Vivaldi, then Britten, then Bach, then...", she said "First we do "tra la la", then we do "la-di-da-de", then we do...". I held my head in my hands to think that after all these years, music for him is STILL 200% about the sounds, and only the sounds...all the rest is just so much fluff, and totally unworthy of his consideration, obviously!