Hello, it looks like your step son has lesion in his calf muscles. The superficial calf muscle gastronemius is attached to d deeper muscle through a layer of tissue. If this tissue is injured or damaged, it might produce the abnormal motion of the muscle as in your condition. Check the degree of abnormal movement with the opposite normal leg. Give him syrup zincovit
to assist in faster healing. Ask him to rest that leg and prevent over exertion. If symptoms persist after a week, it's better to take an MRI
and further hospital management though i am sure it won't be necessary. Thanking you and have a nice day.