Hi!
thank you for your concern :)
I've also done my research and concluded that vegan diet is pretty doable, even for small children. In fact, the American Dietetic Association confirms that well-planned vegan diet is appropriate for all stages of life, including infancy, adolescence, etc. Here's the link - https://www.ncbi.nlm.nih.gov/pubmed/19562864 .
I think you did your research on sites which may be biased against vegan diet.
I read vegan dietitians' sites for specific information, they explain everything in detail on how to do it. Maybe you will think you don't wanna trust vegan dietitias as "biased", but I read only the most trustworthy ones, who never sugarcoat anything and don't portray veganism as a super wonder diet, they show only approprite research.
Anyway, I'm not the first one, not the last one with that. There are plenty of vegan kids out there and they do fine, otherwise there would be so much of noise on that topic.
Also, living in Russia, not a particularly vegan friendly country, I don't think he'll be 100% vegan, but vegetarian for sure.