You can probably connect the dots here to guess what he wants you to do. And apparently there's a whole bunch in the Fort Strong armoury guarded by super mutants.

Now that you're acquainted with the main personnel on the Prydwen, return to the Elder and he'll explain to you that he needs nukes. Note: You will be facing super mutant enemies exclusively during this mission, so arm yourself accordingly (energy weapons). Triggered by completing Brotherhood of Steel quest 06 - Tour of Duty