AIRPLANE for quite some time...he just does it a little differently. Instead of extending the pinkie and thumb, he just extends the forefinger - which we mistook as pointing. We realized that he was signing airplane when we spoke about one indoors and he made the sign. He clearly wasn't pointing at the plane in the sky indoors, he was signing!
Apparently this is a common approximation. In What Do You See? of the Signing Smart series (fantastic interactive books for babies which show a simple sentence along with a picture of both a baby and an adult making the sign), the baby makes the AIRPLANE sign using only the forefinger as well. I better keep my eyes peeled for more signs!