Following is going to be very hard unless you have a zip line, helicopter, or you bike behind him, and none of those are all that easy. The hard thing with DH biking is they cover so much diastase in such a short time, and seeing the whole line is really cool. If there is an area where you can see for a ways, you could go towards the middle or some significant feature, then zoom way in when he is up top, zoom out when he is coming down, the back in (only with a real video camera though). But without a whole team and multiple cameras, it can be very hard.
heres a few segments showing DH that I really like from roam
this on has new music, I wish I could fins the original, but its also slightly more FR. Shooting DJ/FR/Bike Park antics would be much easier
noticed I made a few typos, but anyways the hard part is that nearly all of the non-headcam shots are very brief, and while you do see the whole run, it is not all in one shot. You could try to film it similarly to a rail jam, like pick one or two features on the course, and try to get multiple riders going over these features, just an idea