My advice is this:  His written promise to pay is NOT really a security.  If he breaks his word and walks away from the debt, you have to sue him to win a recovery.  If he has any claims against you (or defenses to the promissory note) then he'll get to raise those in the law suit.

It would be better if you have an attorney draft a Confession of Judgment, where he agrees to pay the loan in full - just like the promissory note - but in the event he breaks his promise, you file the lawsuit and immediately file the confession of judgment.  No trial, no more expenses.

If he's serious about wanting to take on this debt, then tell him that the Confession of Judgment is what you'll accept as his written promise.  Besure you have an attorney draft that Confession, ok?

Good luck to you!