How can I the cosigner take the tax deduction on my grandson's Sallie Mae's debt when I make all the payments?

You can only deduct the student loan interest if your grandson was your dependent when the student loans were taken out.  You will have to enter the information in the student loan section of TurboTax.  If the 1098-T is not issued in your name, you will want to make sure you keep the information with your tax records- records showing you make the payments (bank statements).