Thank you for contacting Justanswer.com. I am sorry to hear of your son's difficult situation.
Oral agreements are enforceable typically, but only if your son has evidence (including his testimony) regarding the terms of the contract to provide a basis for a court to enforce it.
If so, he can sue the roommate for breaching the agreement in small claims court, depending on the amount in controversy. The maximum he may sue for in HI is $3,500 in small claims. Otherwise, he would typically need to retain a local attorney to represent him in the state court.
If he does not wish to file suit, then his best option is typically to give 30 days notice of his intent to move out to the landlord since he is a month to month tenant, so he is no longer liable for rent.
All the best to you in the New Year!
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