With the help of my associate, it's theorized that Elder has been able to regenerate due to the elder berries he was consumed in large amounts. They help with the immune system, and since Elder is under a lot of stress, both internal and external, very often, his body's immune system is constantly on high alert. That physiological response is also why he's often so hungry and burnt out. Once he finds himself in a situation that doesn't call for frequent self-risk and is able to accept that he's safe, his body's immune system will be able to act at a much more normal pace.