Tracking Key Performance Indicators (KPIs) can help gauge progress and effectiveness of training. Here are some KPIs you might consider:

1. Squat Strength: Measuring the maximum weight your son can squat can be a good indicator of overall leg strength. Regular testing (e.g., every 2-3 months) can show improvements over time.
2. Vertical Jump: This tests explosive leg power, which is crucial for hockey for quick starts and agility on the ice. Measure how high he can jump from a standing position.
3. Balance and Stability Tests: Since ankle strength is critical for balance and agility in hockey, tests like the single-leg balance test (timing how long he can balance on one foot) can be informative.
4. Speed Tests: Short sprint tests, like a 30-meter dash, can indicate improvements in leg strength and explosive power.
5. Agility Drills: Time-based agility drills, like the T-Test or shuttle run, can measure improvements in leg and ankle strength as they relate to on-ice maneuverability.
6. Endurance: Endurance tests, like the beep test or a timed distance skate, can be useful to gauge the stamina of his leg muscles.
7. Flexibility: Regular measurement of flexibility, especially in the legs and hips, can help in injury prevention and overall performance.

Incorporating these KPIs into a structured training program can provide objective data to track your son's progress. Remember, it's also important to balance strength training with skill development, on-ice practice, and rest periods to prevent overtraining. Regular consultation with a hockey coach or a trainer specializing in youth athletes can also ensure that the training regimen is age-appropriate and effective.