As long as he has his own signature or seal on the IOU, it is legally binding. If he doesn't pay off after negotiation, you can take the debt as the legal basis and bring a lawsuit to the court within two years.

The statute of limitations for IOUs is 2 years. If the repayment date is indicated in the IOU, the limitation of action shall be calculated from the date indicated. If the repayment date is not indicated in the IOU, the limitation of action shall be calculated from the day after the debtor issues the IOU.