I see this situation far too often. It is a shame when children take advantage of their parents in this way.
Your remedies depend upon the documents you signed. In addition, the interest rate on this loan seems to be quite high. This is a bad loan from beginning to end.
Since you are liable for this entire loan, you need to take a more aggressive approach to the situation in order to minimize your losses.
I urge you to get legal advice on your options. One of those options would be getting a judgment against your son and garnishing his wages.
Contact me for legal advice.
Robert J. Spitz
909 395 0909