If you want to do something down in OC, Old Vine Cafe in Costa Mesa has a fabulous $65 vegetarian prix fixe menu that will keep everyone interested. They also have even more great a la carte veggie items on their dinner menu.
Otherwise if you want something in LA, I'll second the above suggestion for Melisse in Santa Monica. Another good LA option is Osteria Mozza in Hollywood for awesome Italian dishes.
It depends how upscale you want to go. I am vegetarian, but tend to shy away from the fake meat and tofu substitutions that many places offer. Instead, here are a few places that don't substitute for meat, but are good in and of themselves.
I think the best for an 18 year old's party is Cobras and Matadors in West Hollywood. They seem to have changed their menu and offer a lot of vegetarian stuff, so much, in fact, that we leave stuffed every time. Also, it will likely make your son feel a bit older and cooler!
Pretty much any Italian, that's not frou-frou, will work. My hands down favorite is Girasole on Larchmont. One of my meat-eating friends said it was the best osso-buco since her grandmother's. My family members who are vegetarian were overwhelmed because it was one of the first places that actually offers more than 2 token veggie options, and had to unbuckle their belts when they left.
For pizza check out Riva in Santa Monica-the food is good and it's a casual, upscale atmosphere.
I seek that many people here are fans of Akasha, but every person who has grown up vegetarian (that I know), never wants to go there again. It's just not tasty food...it's meh, in my opinion.
if you can afford it, and if you can snag a reservation, i'd go to gjelina in venice.
chaya venice offers one fabulous vegan entree and a few vegetarian starters.
a number of good vegetarian (not vegan) choices at antica pizzaria in marina del rey, which despite it's name offers some very good italien food and despite it's location in a shopping mall offers an attractive restaurant with tablecloths in which to enjoy the food. be aware though, that the owner comes from naples, italy, and makes neopolitan-style pizza so the crust is VERY thin. he imports his flour from italy.
for thai food, ayara thai cuisine has a section of their menu called 'FROM THE WOK;" virtually everything in that section can be prepared with tofu.
26 beach restaurant will substitute or add grilled tofu for any of the meat/chicken in their entree salads (almost all of which are terrific), in addition they make a good homemade veggie burger.
houston's, the chain, also makes a great homemade veggie burger.
if everyone in the party is willing to forgo animal for an evening, shojin japanese restaurant on alameda street offers an amazing array of delicious japanese vegan organic food. also, it is a very pretty restaurant. here is a link:
another option for japanese food is to reserve the large private back table at musha in santa monica. they serve izakaya, japanese small plates, and is all-around fun. they offer many vegetarian dishes and make their own homemade tofu that has a consistency more like ricotta cheese than the big blocks of tofu you see in the stores.
if i were the one making the decision for you, though, i'd pick the persian restaurant, SHAMSHIRI GRILL on westwood blvd. they have an entire section of their menu devoted to vegetarian dishes and , in addition, many of their starters are vegetarian. they also offer tons of skewered meat, poultry, and fish dishes. their price/quality and price/quantity ratios simply can't be beat by any other attractive restaurant offering sit-down meal service.
I don't think I'd suffer through the poor service at Gjelina for a special occasion ~ food's awesome, but only if you want to put up with attitude.
Hal's on Abbot Kinney, almost next door to Gjelina, is a friendlier choice with options for everyone, including vegans.
Also second Akasha in Culvery City.