Extortion is a criminal offense, and applies to a different fact pattern, so no. Unless you son is/was a minor there is probably no right to his claiming an offset. You can sue in on the debt.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Depending upon the amounts in the money you loaned, I may suggest that you explore filing in small claims court. As noted by the previous attorney, absent any "offsets", the claim by your son is probably seperate from the claim by you against this third party. So, select an attorney of your choice, call him/her, and have that initial "comfort" conversation on how to best proceed with this matter.
Shawn Jackson ESQ. (707) 584-4529
Business Development Attorney EMAIL: Attorneys@CaliforniaBusinessDevelopment.com
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Sue him on the money he owes you. Whatever your son owes him is separate from what he owes you. Get an attorney.