Can my elderly parents get out of having cosigned for their grandson's Sally Mae loan?

Unfortunately, the answer is likely "no." A complete and final answer would require an attorney to review your son's promissory note to determine the terms your parents agreed to and try to determine if there is any way to break them lawfully. If your parents are getting harassing calls then I assume your son has defaulted on his loan. In this case, your parents are as "on the hook" as your son is, and unfortunately Sallie Mae will likely not be in any mindset to renegotiate a loan that has been defaulted on.

I would recommend speaking to an attorney (or encouraging your parents to do so) that has experience in debt settlement and loan negotiation. A skillful attorney may be able to convince Sallie Mae to rework the terms of the loan to make your parent's payments less onerous.

NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.