The loan agreement would be legally enforceable in any manner you write it as long as it is clear, concise, and agreed to and signed by both you and your son. You would also want to execute a promissory note.
But the reality is that if you have any anticipation of getting paid back any time soon, you should think twice because this kind of loan to your son is likely not going to get repaid. If John defaults will you sue him? And when he gets out of college and starts to earn his own way, will he have the money to make payments to you? Or will he need whatever money he makes just to survive?

