I am very sorry for your situation. Yes, there are things you can do.
If you demand to view the tapes, they'll likely say no, and they have no obligation to show you the tapes. That said, there is a way you can force the prison to preserve them and let you view them.
You need to send a letter (email would be preferable of possible) to the warden, the superintendent of prisons, and any other individuals you can reach in positions of authority in that prison and up the chain, indicating that you and your son will be suing the prison assault, maltreatment, and civil rights abuses. Tell them that you are giving notice of your intent to sue, and that you are demanding that key evidence, the tapes, be preserved for trial.
If you start a civil suit, you'll be entitled to view and examine all evidence. If you've put them on notice to preserve the tapes, and they destroy them, then you ask the court to give an adverse inference instruction -- meaning that they destroyed the tapes intentionally because it proves their guilt.
You'll need an attorney to file suit and litigate. In this type of case you'll likely not pay anything, because the attorney will only be paid if you win.
But, you should immediately send out those letters / emails to protect the evidence.
That's the civil side.
If your son is being abused, you / he should file complaints with the attorney general of your state, as well as with the US Dept of Justice. You can find the instructions and forms for doing so on their websites.
They will investigate and bring any necessary charges.
I hope this helps.