OP I would be worried about ds being overly influenced by this child and others in general. Lots of discussion about our family doing, eating and living according to your values, beliefs etc. and not allowing others to dictate his behavior, diet, or enjoyment of life at school, home or anywhere.
Families are different and that is wonderful for each family, but your family doesn't dictate what goes on in friends life or household and friend should not in turn.