Learning how to go fast takes practice. With apps like Strava, tracking progress is pretty easy. I would recommend planning out a local route to use as kind of a test track. Once a week (or more, if so motivated), have him ride that route with the intent to do it faster than he did before. Obviously, having the endurance for the duration of the race is also important. 2-3 hour rides at a easy-moderate effort level would be good to work into the routine, as well. If you're already in contact with the local coach, maybe he/she can offer some more guidance, especially things that are specific to your area. The coach might also be able to help connect your son with riders of a similar age to ride/train with.
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