Does the university have a process for grade grievances? Your son could start that process.
I would check with the registrar to see what the consequences/possibilities are if the deadline is missed. I have seen the registrar provide a letter stating that as of a certain date, all of the requirements had been completed even if the degree wouldnt be conferred for a few months.
It does seem like you are at the point where you need to elevate and name names. Let the Dean know you are this point.