Alcohol intake? How is his sleep? Getting sun daily? Mental health? Food is important and can make a big difference but so are all those other things. Get all of it right and you see significant change.

As far as diet goes, IME just eating more meat (showing by example) and talking about it tends to catch on to the people you are around. They start to think, ask questions, maybe read a book. and eventually try to get a little more meat in their diet and see how they feel. So, I think you are doing all that and I think it will probably help him. It seems to me it is hard to get all the nutrients you need eating mainly vegetarian. It is better than processed foods, but worse than eating 80%+ meat.