DD has been doing that for months. She seems to get a kick out of the way it feels. Then of course once we acknowledge it she keeps doing it. But she is starting to outgrow it and is now nodding yes (even though she doesn't quite understand what it means)
Try giving your son more foods that he can feed himself so that you don't have to rely on spoon food as much. He'll probably enjoy feeding times more when he has all the control.