You work on it by locking the wrists/elbows and "reaching out" with the shoulders.
Leaning forward a little (at the waist) is always a good idea, but isn't a "make it or break it" issue as far as his arms rising up. Pushing the shoulders forward (as in "reach out" with the shoulders) is what's needed.
Arms rise up because the muscles in the arms are too relaxed. There's such a thing as "too tense," but there's also such a thing as too relaxed.
Leaning forward a little (at the waist) is always a good idea, but isn't a "make it or break it" issue as far as his arms rising up. Pushing the shoulders forward (as in "reach out" with the shoulders) is what's needed.
Arms rise up because the muscles in the arms are too relaxed. There's such a thing as "too tense," but there's also such a thing as too relaxed.