Flat foot is a temporary deformity caused by muscular imbalance and weakness, aggressive physiotherapy is recommended with shoe orthotics, sometimes taping of foot and ankle joints is helpful too. See an orthopedic to discuss treatment approaches if it can be resolved by physiotherapy or it will need surgical intervention.
Next Steps
Consult an orthopedic and PT
Health Tips
Get him shoe insoles with higher arch support, try it for a week or so, it could be painful for initial couple of days. If pain persists for more than a week then discontinue. Some time they work and some time they donât and that should be ok. Good luck.