16 is pretty much off the scale. I'm about 10 (depending on W) US/Canada- high instep- and I can barely find shoes big enough to fit in some countries.
Germany might be a good place for him to go shoe shopping- at least they have sizes of (relatively fashionable) women's shoes I've never ever seen elsewhere- up to women's size 14 or something like that. Das boot! I don't know anyone with feet that gigantic, just an clinical observation.
Best regards, Spehro Pefhany