Not being sure of the time since surgery....conservatively, there's almost no harm in having your son's physical therapist guide you toward some braces (orthoses) that will keep his feet aligned for longer after the surgery. Only you can decide how much of a problem or concern his current gait (manner of walking) is, and whether it is worth seeking some local help.

All the other replies-suggestions will likely not cause any harm, but walking in the grass may not be enough. You obviously are willing to do everything you can to benefit your son. He is blessed with caring parents.

Barbara, OT & PT