i became a veg. (no meat) in highschool around the age of 14/15. it wasn't for any health or animal rights reasons. it was so i wouldn't have to eat my mother's cooking. (honest) every bad cooking joke out there, she was the cause of it. veggies and rice, peanut butter, breads, and fruits were a safe way to live during those years.
i do eat meat now (very little red and not by product foods ick). every time my mother sees me eat meat she says " i knew it was just a phase" my father just laughs because i have told him the truth, he says it was the smartest thing i ever did. my brother is just mad he never thought of it.
to the OT- if ds wanted to become a veg. (and i knew it wasn't to hide from my cooking, (he does tell me i am an awesome cook so i should be safe), i would say yes but, have some ground rules as to what foods he would have to start eating, to make up the difference to what he was giving up.
there will be plenty of battles to go through, and you have to pick and choose them. this is one i would let them lead in.