When I said knowledgeable vegetarian I had in mind a person who knows how to provide meat substitutes in order to keep a nutritive diet.
As far as I know it's not as simple as removing x and y from the diet, especially when meat is a main source of protein. If I had the knowledge to support that decission fine, but if I didn't I would probably ignore the request and keep serving meat. Now, it would be a tragedy if DS decided to stop eating meat, with him being allergic to fish, eggs, nuts, soya, and most legumes, I would be in a serious problem to make up for it!