P.W.
Someone will probably have a better idea than mine but:
I am not a proponent of spanking, except in areas of extreme danger, such as this. If you are not a spanker, a swat on the butt over this might send a pretty clear message.
Even if you don't do that, for something like unbuckling the whole world should stop and some severe repercussions should happen immediately, so that he gets the message.