K.E.
Have no fear - this is totally normal. Most likely, the first time he did it, it was an experiment and he enjoyed the reaction he received, so now he'll do it a lot! My little guy loves to use his eyebrows to be very expressive because of all the ooohs and ahhhs and chuckles he hears. As another mother said, he'll move on to other expressions soon. Enjoy this time while it lasts because it's gone all too soon. :-)
K. E.