[tomkat]

Nah, he’s pointing with his index finger.

Look again:

             

[Boogieman]

Yes, it is definitely the index finger. You can tell by looking at the knuckles. The longest knuckle is the one connected to the middle finger, and in that photo, you see the knuckle below the pointing finger extends further than the knuckle above it. Therefore, the pointing finger cannot be the middle one, it can only be the index finger.

What is above it, that looks at first glance like the knuckle of the index finger, is actually his thumb, with the tip folded behind his index finger.