You hold the Boost Button, I believe. I think the game flashes the required button at when when you start falling, but it IS brief. You'll need to constantly hold it to catch up with him and deliver the hit, if I recall correctly.
Since I was using an XBox 360 controller, I held X. Got me to him just fine while avoiding his shots.
"Ironically, the word mature when applied to games tends to have a very childish connotation." -Erik J. Caponi of Bethesda Softworks