Did you get any collateral as security for the loan? If so, you could foreclose on the collateral. Otherwise, send a demand letter and sue him. Unfortuately, if he is judgment proof (meaning he has no assets from which a judgment could be collected) suing him is not necessarily going to get you your money back. Good luck.
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See if you can get him to sign a promissory note and have him pledge some collateral to secure repayment. Offer a discount on the amount owed or a reduced interest rate to entice him to sign the note. If that fails, go with Guy's advice above. You will have to sue it out in district court in the county where he resides and it is your burden to prove the terms of the oral agreement.
Just beacuse you had an oral agreeemnt, does not minimize the fact an enforceable contract may have existed. Based on the limited information you provided, your recourse most likely is to allege breach of contract and unjust enrichment, filing the action in District Court. You will also need to consider that even assuming you can be successful in your claim, what would happen next? For example, would it just push your son in law into bankruptcy where the debt may be discharagable. If not, would he have any collectible assets?
You should consult an attorney who can walk through these and a host of other issues you would want to consider before strating a lawsuit.
The information provided herein is for informational purposes only and is not intended to create an attorney-client relationship or provide legal advice.