How do you make semi-vegetarian meals?

My oldest son (17) has decided to be a vegetarian. In looking at the types of vegetarians, he is probably a semi-vegetarian. The rest of the family is not. I have no problem with his choice. However, how do I make meals which are both vegetarian and omnivore friendly?

Tonight we had quesadillas and vegetables. His were just cheese and the rest were steak and cheese. He is not real strict in his rules as this was introduced by a girl (surprise!) and is more about animal cruelty in mass meat production than due to health/diet concerns.

I have a few ideas on how to accomplish this, but suggestions would be appreciated. Thanks!