Fort Drum scans your Military Retiree card at the gate as you enter the Fort. Your brother may have to get a pass. Better check with the Fort Provost Msrshal Office to see what documents are needed before you go there.
As a retiree you should be authorized commissary access. The ID card is scanned at the register at time of purchase. You can bring a guest in with you, however you must be the one purchasing for your own use.
Thanks! Your answer is awaiting moderation.