I would try and explain to him about where you buy your meat etc and give him as much info as you can but you will probably find this is a phase he will go through. 6 years old is extremely young to want to do this and I bet in a few weeks he will want to go to Mcdonalds again! Just go along with it, maybe his friends are doing it too or he might have heard someone else saying bad things about meat, either way just because he has decided he doesn't want meat now doesn't mean he won't change his mind and it is not really that big of a deal, it could be worse!