At 18, he might enjoy Geyser Point and Trader Sam's - even though he can't drink, he may regard TS as a cool atmosphere.
I would work in one World Showcase meal for a first timer. If never done hibachi where you live, Teppan Edo could be something different. Via Napoli is also popular, as well as Biergarten (especially if he is not picky on food, since German cuisine is a bit polarizing). Chef's de France or Monsieur Paul used to be trip staples for me, but I feel like Chef's quality has declined and the newer prix fixe menu at Paul is way overpriced.
There are plenty of other great options from signature to causal already mentioned, but I would base it on where you are staying or resorts you may want to see. WDW is so large and you'll be doing so much traveling around, it's best to make meals as convenient as possible. Check out the Boardwalk and eat at Flying Fish, want to see the Poly, so try the new Wailulu, etc.