Well the whole point of being a co-signer on the loan is so the bank has someone else to take responsibility for the loan should a risky debtor (like a student) default. You really can't get off the loan at this point, but if you are sued you
You should consult an attorney who specializes in debtor-creditor law (such as me). I have good news and bad news. Bad news is, you are still the cosigner and bear responsibility. In fact, the lender can pursue you exclusively while ignoring your son. Good news is, if you enter retirement and earn only certain kinds of income (such as social security and certain pensions), your income may be beyond the reach of creditors. But through non-wage garnishment, a judgment creditor can attempt to reach non-wage assets, and a judgment lien can attach to any real property you own.
The easiest way around this is for your son to get a forbearance or deferment, or otherwise make suitable payment arrangements.
I'd highly suggest you consult with a consumer rights attorney. Depending on the type of loan (private or federal), you may have a number of options.
Some private loans have a co-borrower buyout provision in the terms of the loan. Other private loans have special insurance provisions that may also provide some relief.