I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
You have to be able to go about reasonable methods to give him notice (publishing in newspapers of his last known address, etc). Once you cannot find him, after a certain time period, then you can get a no fault divorce based on abandonment or some similar cause of action. However, what state are you from? We need this information to help you with more specific answers.