As others have said I would respect your son's feelings and let him try eating a vegetarian diet -- he's showing great compassion, empathy and thought and it would be a shame to stop him.
I raised my two DC on a vegetarian diet. They are now 10 and 7 and perfectly healthy. Getting enough protein is pretty easy -- dairy, eggs, beans (including baked beans), hummus, lentils, nuts (including peanut butter), peas, tofu, meat substitutes... You need a combination of all of these to get full protein.
Getting enough iron can be trickier -- green vegs, fried fruit, lentils, fortified cereal etc.
Getting enough B12 is essential, but a reasonable amount of dairy and eggs covers this.
Your DS may well change his mind in the future, but I would support him for now.