My neighbor has a mix breed adult dog with both a sensitive stomach and sensitive skin and coat. He is a Pit Bull, Boxer, American Bull Dog mix with lots of energy but is VERY picky and has really bad gas. My neighbor doesn’t want anything with chicken, beef, lamb or buffalo. She also wants the food to be grain free and all natural. He is on Simple Wellness Lamb and will not eat it. He also has tried Merrick Grain Free Buffalo and Sweet Potato and also will not eat that as well. After he eats he eats lots of grass. Please help!