Again, I would not be surprised if the athlete is overtraining. Check the Daniels book section on what he calls "Hypertraining". If it was an inability to push through pain, would the athlete be able to perform the workouts?
It may also be a case of nerves or confidence. If that is the case, give him some confidence building workouts, don't lay into him for being a pussy (thereby continuing the downward spiral).
If you really think it is confidence or an inability to "tough it out", then give him some cues to use during races. Tell him to look for the pain and embrace it when it shows itself. It is much easier to back off at that point, but if you attack it, sometimes it can shake you out of a rut. It is important to let athletes know that if they are hurting, chances are the competitors around them in the race are also hurting. The worst thing you can do is assume that everyone else feels great, because they don't.
If none of this works, then shift the pressure. Let him know that it is easy for him to puss in a race, but his teammates are out there making it hurt for each other. The least he could do is return the favour (don't tell him that 'you' want him to run harder, but rather, that his teammates want to see him succeed).
If none of these things work (over an extended period of time, not overnight), then he is likely overtraining... or he has a healthy case of "rectal-cranial inversion"