Hi M.,
Does your son have bis 2cd year molars? My daughter is 17 months and started doing the same thing... out of the blue. For a week I kept telling her to take her fingers out of her mouth ... and then she started with her runny nose. She ALWAYS gets a runny nose when she cuts teeth. So, I put my finger back there and sure enough I felt some ridges. 2cd year molars. (I know it can be early for her to get them, but she had her entire 1st year set of teeth by her 1st birthday.)
If it is not that, the only thing I can think of is he is mimicking someone elses behavior. We all have our little quirks... I twirl my hair, my sister chews on a pencil. Is there anyone he is exposed to that will put a finger to their mouth or something similar when they are thinking?
Have you tried putting tape on his finger? It might deter him from wanting it in there.