First off, is this a privately or federally funded Sallie Mae student loan? More specifically, what is the name of the loan program that you are cosigned to?
With most cosigned student loans the cosigner can take the student to court if they don't hold up their end of the contract. However, I'm not sure if the loan needs to be paid-off by the cosigner in order to take the matter to court.
Do keep in mind that as a cosigner, you are still legally responsible for the loan and will be credit reported and subject to any negative impacts that will accompany the loan should it default.
Is there a reason that the loan is not being paid? If the student is unemployed or unable to make payments he might be able to request a short-term deferment or forbearance on the loan. As a cosigner, you won't be able to access the same deferment/forbearance options, but they still might be worth investigating.