If the government vehicle that caused the damage is self insured your son should first request of him the information needed to file a claim. If the driver is unable to explain how to do so then your son can contact the Oregon Insurance Division. They have a consumer department that should be able to instruct you how to file a claim in Portland, OR against a self-insured government vehicle.

It is likely you will have to file a Notice of Claim within a certain period of time to get the process started. Because of this being difference than a normal insurance claim make sure your son gets his claim filed within the specified time frame so that his claim is not denied.