What is covered with Medicare's and Medicaid's prescription drug plans?

My brother is 54 and on Medicare disability and Medicaid. He has a AARP RX card for prescriptions. He doesn't remember how he got it or what the benefits are. When he presents his card with his Medicaid card at the pharmacy, he usually pays around $2.40 for each refill. He recently received a check for $117.00 for all of the prescriptions that he paid for this year. Does he not need the AARP card? Are his medications covered by Medicaid? Also, I found a payment book for AARP. I am his POA and have recently taken over his financial responsibilities. My question is, what is covered on his RX plan under Medicare and Medicaid? He is on a spend down for his medical. Thank You!