Can eating only grass fed beef and pork change your health?

My stepson has a friend who raises grass fed cows and a lady who raises grass fed pigs. He now buys all his meat from them. He has three sons each over six feet tall and says they really eat a lot of meat. Now he has found a local dairy that delivers organic milk to his doorstep. He buys ¼ of a cow, has it cut to order and puts it in their new freezer.
Would you suspect that eating this way would benefit the health of his family quite a bit ? They were healthy before he thought of doing this.