My husband has endo surgery on the 17th of June. Funny thing is no one mentioned how the skin on the under forearm would lose the muscle. It just kind of hangs there. The doctor said this is normal and would take a while to come back but what caused it. Lack of using the arm or from being cut? Does it come back?

It is most likely from not being used. It's amazing how fast muscle can be lost from disuse following injury or surgery, but it will return. Does your husband have exercises to do?