Hi!
Eating specific things cannot make you anorexic. If your grandson eats his stomach fill on things he likes he is not anorexic. However, yes, his problem needs to be addressed. Eating too much of one type and not eating another type will usually cause deficiencies. Simple award techniques like eating one thing of your choice and one of his will get him something usually works with a 14 year old. If these tricks don't help, then you need to discuss this with his pediatrician and school counselor. Take care!