There should be beer in the castle in the room north west of the main throne room. Some people also reported finding the beer he wants in the guards' barracks.
Try dropping non-beer/ale alcohol from your inventory.
Ben is on the castle wall, isn't he? I think I recall someone saying that after they left the castle and did some other stuff, they returned with some of Rimmer's ale, talked to Ben on the wall and were able to complete the quest.
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