If your brothers' group like Techno, then Tresor or Berghain are a must. Both go on until 8am in the morning. However, large groups of lads (especially on a stag) will not have a chance of getting into Berghain. Better to split up into smaller groups. Nightlife is great and the beer is even better! Our group all agreed that Franziskaner weissbier was as nice as any beer we had ever tasted! Costs €3 a beer for a large one.
Berlin is great. They will enjoy it wherever they go.