First you can go to the DA with the certified letter and the check and you can get a warrant for his arrest. You can also file a civil suit against him to recover the money, but you will have to know where he lives in order to file in that state and have him served, so you may need to wait until he is arrested or you can simply check his driver's license through the state DMV
where he resides and the information in their database is public information and you should be able to get his address from his license.
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