Mountain bikers tend to have cornered the market on clavicle injuries. They are going downhill at a fast rate of speed and often find themselves going over the handlebars when they munch it. Landing on your outstretched hand while at speed creates a shockwave that travels up your arm and damages your shoulder or clavicle.
Clavicle injuries are most often quite stable and easy to manage. Pain is a major consideration and pain management should be high on your list of treatment protocols.
Once you have completed a full patient assessment you can begin treatment. Whether the injury is a separation or a full fracture the treatment is the same. Grab a triangular dressing or cut up his spare tee shirt into long strips.
This is called a clavicle splint and a quick google search will show loads of variations. Basically run two strips over the shoulders and make an X at the small of their back. Tie it tight enough to make their shoulders rotate back. It makes them look like they are standing at attention.
This treatment will provide some pain relief but more importantly will stabilise your casualty and make evacuation easier.