They can sign papers saying they discharged against doctor's orders. That's what my dad/grandpa had to threaten with my grandma not that long ago when the hospital dr would not discharge her (her PCP does not have priviliges at the hospital she was at).
My grandma has been going through a similar scenario, except we know exactly what sparked her episode - Abilify.
It took two different hospital stays and a nightmare-ish stint at a Rehab facility before things have finally started to stabilize for my grandma (after a total of 2 months).
I would think your parents would have every right to refuse certain medications. Have they tried to contact a patient advocate?
Also, they can call 211 here in FL to find a patient advocate or find out what rights they have concerning your brother's care.
Still praying for all of you.