Panda321

I have read that you can try to submit a notarized affidavit, but I have my doubts.  Perhaps a divorce attorney could help find the official vital record.  You can also get some info by Google search "How To Get a Copy of a Divorce Decree".

This is where you need to be a bit persistent. They have the  records, they just don't want to look for them. They will not be digitized, so the person who you speak to needs to look them up in the old file log books. Then they will have to either send for them off site from storage or dig them out themselves. They are most likely stored off site. 
 
No need to be rude or pushy, just plead with them, pull on their heart strings begging for them to help you find a way to track them down. 

The State Department says that divorce decrees are attainable from Cambodia, that there are no alternative documents, and no exceptions.
 
https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country/Cambodia.html

Maybe the information on how to obtain the document will help you.  Good luck!

Your Mother can sponsor him once she is a LPR

Without solid documentation that the step-parent relationship was established before you turned 18, I'm afraid you're out of luck.