I agree with Mr. Earl and suspect that you are correct as to the time limits. In addition to law enforcement, you might see local counsel to determine if there are any causes of action under WA law.
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.
Though your question is posted under "White Collar Crime," you say you want to "sue." As to the almost six years passing, the statute of limitations is three years for most felonies of this nature, so it is probably too late for any criminal case. The same for any civil suit based on oral promises.
For written agreements, the statute of limitations is six years, so if you have a written agreement between the two of you that he violated, then you have a chance, but must act fast since it was "about" six years ago. Under the "discovery" rule in civil cases, the six years may only start to run from when you found out he was embezzling the funds.
If there is no written agreement, it appears your are most likely out of luck. Always act as promptly as possible in pursuing a clear legal wrong.