Well let’s put it this way! If he thinks that’s where his thumb is supposed to be then his name must be “Wrong Way Corrigan”! Butt… I mean but perhaps he has discovered a better way? Hey, don’t knock it unless you have tried it!
Of course there is the possibility he never outgrew thumb sucking as a child. Though when it comes to “finger licking good” I can think of something a bit better…