I don't know if this would help or not, but if it is a depth perception thing, walk the plane out to where you should be on a straight line for landing with nothing in between. Go back to where you fly from and look at the size. Maybe have your son hold it in the air at different angles to get the size in your mind. (Oh make sure no one else is flying at the time or the field is empty.) I don't know if this would help, but it would give you a perspective from where you fly from to what size the plane appears to be on approach.