Generally speaking, you cannot reopen a personal injury case after it has been settled.
Personal injury settlements are designed to be final. If you and the defendant agreed to the settlement amount, you likely signed a form that released them or their insurer from any future claims arising from that incident. Most insurance companies and defendants will not go forward with a settlement until the plaintive (the injured party) has signed such an agreement. Even if you did not sign a release, there may have been a verbal agreement in place that has the same legal weight as a written and signed agreement.
Every personal injury case and settlement is different. Without knowing the specifics of your particular case, all we can say is that there is a good chance he will not be able to reopen it. There are always exceptions. For example, if an agreement was reached, but one party was deceptive or fraudulent during the settlement process, the case may be able to be reopened.