It might of been a lack of minerals which the meat could help out with....
check out http://www.westonaprice.com for some info on benefits of organic pasture fed meats in the diet. I was vegan for a long time and I have since concluded that it really is not the optimal diet. If your son wants to continue the vegan thing get him to add in a zinc suppliment as well as b-vitamins and hemp protein.That will help his skin as well...