I'd be worried if you said vegan. With a vegetarian diet he should be able to get all the nutrients he needs and then some. It's probably he's cooking/eating wrong - e.g. we are made to eat mostly cooked food. If he's eating too much raw veggies that will have negative effects in time, especially affecting energy levels. So he should learn to cook nutrient dense foods.

Then comes the quality of the veggies. A lot of the supermarket veggies are trash and have like 10% of the nutrition they need to have even though they look great. Switch the source to something else if at all possible.

Combine that with feta cheese or similar and he should be golden!