Perhaps it was Battlefield 1942 with the Desert Combat mod installed? Maybe with a few other mods to fill in spaces between WW2 and the 90s? Check www.planetbattlefield.com for descriptions of BF1942 and the various mods available for it. I can't remember any game that covers so wide a scope except with mods, and your description of the classes your brother had to choose would fit BF1942.