I agree in principle with what lease said.
Important in sprinting is the foot position upon striking the ground--the foot should be cocked, or dorsiflexed--not to an extreme extent, but to a power position.
When athletes "reach" out in front of them with their foot, this advantageous dorsiflexion disappears, or else they would land on their heel with the ball of their foot in the air. The more they reach, the more plantar flexion occurs at the ankle, and the less beneficial force can be transmitted to the track. That, plus all the usual CofM and braking force arguments.
I would suggest that you tell the kid to concentrate on the angle of the foot, and to try to run "lower". I see a lot of kids running too high, and therefore not developing enough power, because they think that the taller they are, the faster they will be! Arm action drives leg action--good knee lift often means good rearward action with the opposing arm and leg, which means good recovery, which can enable a long stride.
I would have the kid do accelerations up a semi-gentle hill, concentrating on looking down at the ground, and on driving the knee forward...the angle of the ground will compensate for the lack of any dorsiflexion he may exhibit.
Then, when his body is somewhat accustomed to the arm mechanic that is required, and the resulting knee drive and recovery mechanic, transfer some of that to the track, and you will have increased stride length, for kids, for the first 30 or 40m.
For increased stride length at speed, I would put the kid on a treadmill, where conditions are controlled, and where he can learn to run lower and dorsiflex, where he can experiment with form without trying to actually go as fast as he can, as kids often do when on a track. Also, if you're inexperienced at coaching or sprinting, you could video him here with a cheap P&S with decent video, and give him some feedback that way--sometimes it's easier to see things on video, no matter how good a coach you may happen to be. Videoing on a treadmill is easy, too.
Also, get him to do lunge-walking, to get used to the idea of knee position, stretch, and force development in that position, while his foot is firmly in contact with the ground. During this movement, get him to concentrate on his foot plant, and to keep his weight over his foot so that he maintains his balance. The lunge-walking shouldn't be done too quickly at all--it's more about stretch, control, and position. Make sure he's not settling on his heels during this exercise, but that most of his weight is supported by the ball of his foot, which he can practice keeping dorsiflexed during this exercise.
Finally, get him to study videos of Jesse Owens, and tell him to work on his treadmill form until it matches Owens' form closely. His cycling was terrific, and with a reasonable stride length, Owens most certainly did not overstride--not only that, but he didn't stretch it out at the end of a race like guys do now, so video of him shows beautifully consistent high-speed form, perfect as an example to show a student.
I could go on, but there are lots of resources available, that give ideas better than I could ever do. Good luck!