dear colleague,
In that case I would make sure he wears (preferably) ankle- high boots with a stiff comfort (heel) to prevent his calcaneus from tilting. if this is not possible ... an inlay ( often calcaneus cup) might be a necessary. By giving him a stable base he might not stumble as much and it should help him. I have seen many children with hypermobile joints do a lot better with good boots.These kids als do well in water- they love it and you can use it to strenghten him. Get hij to swim if possible.
kind regards
esther de ru