Rather agree with NJ. I was going to say that it is unfair on you, Smow and you should tell him to plan and cook his own meals. Someone mentioned 'would he kill fish?' perfectly fair question to ask him. For quite a while I did not eat mammals, but fish and fowl were, I felt OK. We still eat 2/3 veggie meals a week- perhaps you could suggest that option - but honestly, I'd tell him to sort it out himself. A few hungry days should do it. Another option is to use only ethically produced and slaughtered meat , as do I.
He's pressurising you because of his exams. You have your hubby to consider - he helps to pay for the food, son doesn't.
PS You could add that vegetables have been 'heard' (electronically) to scream when pulled from the ground. Sorry, can't remember where I got that one, but it was a scientific study. It's kill or be killed in the world. :)