Try superman starts onto a high jump mat. Also by cueing the arm to extend fully up and out at the sound of the gun may fix the problem. The legs do whatever the arms do. If he is not extending the arm fully the legs may be cutting off early as well.
Superman starts onto a HJ mat? I'm not familiar with what you are talking about I don't think.
stop trying to give cues and get them stronger esp in the post chian, most young sprinters cant get full ext bc they lack strength. some runners are quad runners which tends to hinder them from achieving full ext.
Yes, we've been working on his posterior chain strength for some time now and he is actually quite strong. Also, we are getting close to the state meet so the time for strength development is nearing its end.