HI there,
My DD sees a physio as she was very prem and has low muscle tone resulting in her ankles rolling right over. The physio advised me that babies/toddlers are all flat footed as they don't develop arches until they start standing on tippy toes etc. Anyway, my DD is not walking and has the ankles rolling over which effect her stability. So the physio had her assessed by a pead. orthopeadic person and she will have orthotics to help correct her ankles. If it is effecting his walking and you are concerned, I would go to see a pead. orthapeadic doctor.
HTH.