Can my step-son have Anorexia Nervosa?

He has a new job working at a restaurant serving tables and vomits every time he is there, so he is now working where he is not around the food. He refuses to eat often and when he does he is eats less then his much smaller sister. Just tonight he ate 1 small meatball and about 2 forks of spaghetti noodles. When questioned, he said "I don't want to get hungry or have an appetite." When he said that just after telling me he had to change jobs because he vomits around food, the red flag went up. I am concerned that he is not just under weight, but is actually suffering with Anorexia. Any help is greatly appreciated.