At what age is it okay to let a child start to lift weights to help with sports performance?

Use a trainer. Weightlifting like many sports activities can be beneficial if done right & can produce significant injury if done wrong. Any pre-teen/teen should be supervised by a professional, preferably one certified in the area.Early training focuses on technique & low intensity weights. Power lifting & body building are not recommended until the kid reaches skeletal maturity (final height, b.O., shaves).
His decision. Let him decide. The ohnly supervision required is that the weights are proportional to the age and strength of the child.