i have flat feet...and its only in recent years that it caused discomfort.
i suggest you get a referral letter from polyclinic to see a specialist..nothing major, no need surgery
but what the specialist can do is custom make an insole for his shoes, help to correct the flat feet if possible
too bad im too old to "change'...but for kids still possible.
i got mine done at Changi hospital..
also get a letter from doc to excuse from Running and Skipping
swimming and cycling are good enough for him