It might be from using the stationary bike, the constant friction on the bottom of the foot for however long he is using it. Its not the same friction that you get from actually walking. I know that skin grafts break down if they continuosly rub against shoes or tight or sweaty or wet clothing. There are machines that allow you to do aerobic activites with your arms instead of your legs. If it were me id stay off the stationary bike and really limit exercizes pushing your feet against pedals on weight machines until this thing has been healed for a very long time.