I've been looking at other animations that involve falling/sky diving for what they have done to create the illusion. It's mostly movement of clothes and some limbs. The background should be animated too, with things farthest back moving slower than the foremost things. Here's some of the videos I looked at:
I tried to apply what I saw in these animations into a test animation, just to try out creating the illusion.
Not bad but he needs his backpack (which I don't have at the moment but it's not necessary for the test) and maybe an altimeter to complete the look. I think I'll apply some camera shakes to help enhance the illusion of being in free fall. I noticed some subtle camera shakes in the example animations which I think really helps.