I'm not a lawyer, so this isn't legal advice. For that, you (or your son) need a lawyer. However . . .
Dan's advice is good. Best thing to do is for your son to pay.
It's likely that what the park manager actually said--not what your son heard, but what the park manager actually said--is that he'd file a "5 day notice to pay or quit." That gives your son 5 days to pay or leave. Consult with a lawyer on the difference.
You ask if he can convey the home back to the lender? What do you mean "back"? It wasn't the lender's to begin with. And the lender doesn't want it back. Maybe the lender will accept it, and it's certainly worth a try. But your odds are slim.
So: Your son should see a lawyer.