This is often a side-effect of having a high body fat percentage, or retaining a lot of water. Both of which will put pressure on his veins to reduce blood flow. Reduced blood flow, means less oxygen transport. Arms get starved for oxygen, and they go numb.
However, this can also be the sign of a more serious problem such as a heart condition where the heart has difficulty pumping blood when his arms are raised, or a problem with his blood cells.
My advice is to go to a doctor and get a complete physical with blood analysis done.
Justin
