how do I get out being a cosigner on a student loan

Absent some facts of which I am not aware (e.g. you cosigned the loan at gunpoint), you can't.  When you co-signed the loan, it was for the benefit of the lender, and if you renege on that obligation it is not your grandson, but the lender, you are hurting.  You may feel that your grandson should stand on his own two feet, but the lender didn't rely on your grandson doing that; it relied on your promise that, if your grandson didn't meet his obligations, you would.  While you can't (again absent circumstances not apparent from your email) get out of being a cosigner, you can sue your grandson to repay you if you have to pay any part of the loan.