Here in NJ and every other jurisdiction with which I am familiar, the prosecution (not the cops) determine what the deal is. Your brother needs to ask the attorney whether the prosecutor has approved the cooperation as a basis to affect the plea; what the deal is without cooperation; and, what the benefit of the cooperation is in regards to reducing the plea terms. If your brother is not satisfied with the answers of his current lawyer, he needs to consult directly with another PA defense lawyer to get a second opinion before he makes any ecisions about what to do. Good luck.
DISCLAIMER I do not practice law in your State. This answer is provided solely for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.