He may have tried to take a short cut and ended up getting throttled across the face by a branch, cutting through a ravine he may have done a nose dive and endo'd the bike after hitting a deep rut, if he was thrown off the bike he could have gotten internal injuries that weren't apparent initially, the list of possible scenarios is endless and is pretty much left up to the imagination.
If this kid planned on running off and hiding, or if he planned on disappearing voluntarily for awhile, it certainly doesn't appear that he planned for it very well. Further, why bother to send out a couple of texts saying you hurt yourself if the intention was to vanish without a trace ? It appears to me that he took basically no personal items at all, other than the clothes he had on, a phone, a little bit of money (rumor), and the bike. Even a scatterbrained 18 y.o. kid would want to take along some of his "stuff" if he planned to be gone for very long.......you know.....like a cell phone charger ?
His friends and family need to approach private landowners in the immediate vicinity and ask if the owner will scour his property or let some volunteers do it for him. I think most property owners would agree to it as long as it was done in such away that nothing was torn up in the process.
I noticed there was a nudist colony a very short distance away from where he lived, so I'm going to go over there with a spotting scope, up on the side of a hill, and scan the horizon for a few hours. Speaking of which, if I was an 18 y.o. kid I'd probably be spending a whole bunch of time cutting some dirt up around that nudist place. Just a thought.
Map here:
https://drive.google.com/open?id=1sp8ToRC1oXf4TugFTSFOGfaubZY&usp=sharing