If the bow limbs are set at the weak end of the draw spuctrum his bow might experience more vibration. Todays bows are set up to shoot their best with the limbs bottomed out. Also a bow at say 50-60 lb set at 60lbs will outshoot a bow that is 60-70 shot at 60 lbs. When the limbs are loosened the limbs are driving in a more upward position than a more foward position. If it doesn't overbow him try tightening the limb bolts and see if this cures the problem.