Babies play mindgames all the time! They learn that certain actions provide reactions, and they try themout from time to time to see what happens.
What is happening here is that like all children, your son is exploring his orifices with his fingers, and he has found a reflex action that gets a reaction, so he will play on it. Children do the same with coughing, and evencrying sometimes. It is all a perfectly natural form of development, and no cause for concern. If you ignore his behaviour, the novelty will wear off, and he'llmove onto something else.