Consult an orthopedic for evaluation and get done STEPP test for flat food evaluation

If he had issues With Walking and playing, please visit nearest orthopedic surgeon Need Clinical evaluation

It will develope naturally

Your child has flexible flat foot as arch appears on tiptoes.This is normal at 6 years.

In a 6 year old child flat feet with an arch appearing on standing on toes is called flexible flatfoot and is very common and usually normal in most children the arch develops naturally with growth, often up to 8–10 years of age Since your son has no pain or walking difficulty, no treatment is required at this stage special shoes or insoles do not help in developing the arch and are only needed if the child has pain or fatigue normal activity barefoot play on safe surfaces, and comfortable footwear are sufficient for now

Use custom footwear with medial arch support. Its flexible flat foot, strengthening by physio