Our dec athletes have usually been (primarily) pole vaulters or hurdlers.
The Decathlon is a speed/power event so get him into the weight room and minimize XC.
It's hard in a HS program to train someone to become technically proficient in all the events, so make sure you focus on overall athletic development and learning how to sprint and accelerate properly, which are the foundational skills for speed/power athletes.
I like the advice above about giving him the rudiments of each event, but be very careful with what is taught in the more technical events: discus, jav and pole vault. It's very easy for strong athletes to get decent marks in these events using really bad technique, but as the previous poster noted, the athlete will have to unlearn these bad techniques to progress. The point here is that it's (almost) better to be untaught than to be taught poorly.
From what you've described, he'll probably have the most success soonest in the 300h. Success helps kids love the sport, and if he's going to do the dec, he'll have to really love the sport.
If you can spare him from relays, train him for both hurdles (which will cover the 100 & 400), a throw and either the high or long jump from a short approach, and then see how he responds.
If he wants to do a dec this summer, you can show him the rudiments of the other events in a few afternoons, at least enough to get him over 6' in the vault and get a presentable mark in the other jumps and throws. The 1500 takes care of itself.
But I'd suggest not moving to another technical event or trying to master one until he masters hurdling.