Berserk, Crying Freeman, get him into some real classics, and show him the first original FotNS because thats how it should be. Green Legend Ran, Legend of Crystalis, Vampire Hunter D, DevilMan (make him grow a second pair), Ninja Scroll, anything done by Studio Ghibli (turn him back into a kid again), anything done by ProductionIG, Ghost in the Shell, anything done by Manga Studios...the list goes on. Although Cowboy Bebop (very good), Trigun (ok), Evangelion (shat bricks), Tenchi Muyo(ok), Gunsmith Cats(good), and Goldenboy(ok) are ok jumpstarts. Start him on baby food and get him to steak. Careful of mech/shoujo/mahou animes (please be VERY VERY sparingly on the Gundam).
Good luck...don't make him a weeaboo