if there is a stipulated judgment that means you have to go to court to modify the payment order if you are not able to maintain the $90 per week you agree to. if you go to the judicial website you will find that there is a form for you to fill out call the motion to modify payment order. that's where I suggest you start.
Is your son a co-owner of your home? Or were you a cosigner of his debt? if you answered no to both of those questions and based on the brief summary you gave it would be inappropriate for a lien to be placed on your property for his Debt.