If your 14-year-old grandson is already somewhat conditioned (meaning that he has been participating in a physical conditioning program and is used to high-intensity exercise), then he might be able to tolerate a P90X workout program. However, your question specifically focuses on “bulking up,” which I’m assuming you mean adding muscle mass and strength. For this goal, your grandson should focus more on heavy weight-lifting exercises (such as the squat, deadlift, and bench press) to stimulate muscle growth and strength, rather than high-intensity interval training with body-weight, which is common in P90X workouts.
That said, both of the sports you mentioned that your grandson plays (soccer and basketball) require the athlete to sprint and move quickly, which could be helped by the P90X-style of training. They also require the athlete to produce a fair amount of explosive power, so Olympic style lifting (the clean and jerk and the snatch) would also benefit him.
My recommendation is to have your grandson work with an ACE-certified personal trainer who specializes in youth sports so that he gets a well-rounded, periodized conditioning program to meet all of his needs as an athlete. The P90X program is an OK place to start if he can tolerate it, but there is so much more to training related to becoming a good athlete that I would not stop there. If nothing else, ask your grandson’s coaches what he should be doing during his off seasons to make his athletic performance optimal during the competitive seasons.