Patience. Don't expect too much too soon. If he was sick for a month preop, it will take a month or more to recover. Remember, he is recovering from both a serious illness AND major surgery. This takes much longer than coronary bypass surgery. The pain should get better with each passing day.
So, the idea is to walk within tolerance. Rest when needed. Plenty of fluids. Eat what appeals to him. He will improve with time. You cannot rush it.
Jan