So... the only difference is he is now not eating dairy and eggs?
We do not need dairy past infancy, and nothing says we must have eggs.
Are you worried about calcium? There is plenty of calcium in kale, carrots, and organic vegetables. Protein? We can get ALL our protein needs from fresh fruits and vegetables with a bit of nuts thrown in, without even adding in grains like Quinoa, and the legumes (beans)!
I and my kids are allergic to dairy, eggs, and even many grains. No problem. Actually of everything, there is one thing even my doctor doesn't think we should stop eating at least occasionally - fish, or at least, fish oil - although many thrive with the essential fatty acids from walnuts, almonds, coconut, and flax seed oil, as well as eating a minimal amount of avocado. One of my kids is now getting her Master's degree in dietetics/nutrition.
He does need a B12 supplement or get it in the form of a special yeast, but even meat eaters need B12 simply because our soils are so depleted. I was a heavy meat eater at one point in my life, and STILL required a methylated B12 supplement! So "only vegans need B12" is a total myth.
I have a family member who grew up vegan by choice from a young age (even though her parents were not vegetarian) and is now a beautiful, strong, healthy adult. It caused the whole family to modify their diets and learn a healthier way of eating. This then had a ripple effect in the extended family - not becoming vegan - but now eating healthier and living healthier lifestyles.
And another story... A high school friend became vegan, but ate very unhealthy - so much JUNK FOOD is still "vegan" and that's what he did. He got fat, sickly, and eventually left veganism (but he then ate processed meats and such).
So, we can eat unhealthily no matter whether we are vegetarian, pescatarian, vegan, or regular omnivore. Or... we can eat a healthy balanced diet with good sleep hygiene and adequate exercise on any one of these lifestyles. It isn't "vegan" or not which will decide his healthy eating... it will be his choice. And it isn't just food... but clean water, clean living. I hope he chooses health. And if that is his goal, then kudos to him.