Our children have been raised vegetarian and they don’t need specific vitamins as long as they eat a varied diet. Luckily, our kids are happy to eat most vegetables and things that we can disguise nutritional products in.
Your son should be able to get B12 through his diet. If he eats eggs and dairy products, they are rich in B12, as is Marmite. B12 is also added to fortified cereals (including Kellogg’s cornflakes) and present in soy/ tofu products, which may also be good protein replacements. We tend to stir some marmite through tomato dishes, or add some nutritional yeast flakes (you can find these at most big supermarkets). B12 deficiency is something you have to think more about if he becomes vegan or cuts down on dairy/eggs etc.
We have also found that buying things like seeded breads, encouraging but butters on snacks etc really increases protein intake without much effort. Beans and seeds are also great sources of iron - even baked beans!
Certain seeds are also good sources of Omega-3, which can be difficult to get in a veggie diet. We tend to make Chia pudding, or sprinkle flax seeds on breakfast foods, into sauces or in homemade breads.
We do give our Kids Haliborange multivitamins and vit D as well, as we would if they were not veggie. If you are buying a. Kids multivitamin, do be aware that many aren’t vegetarian and the gum type ones normally have gelatine in them.