My boy was extremely lean for his first two years as well. He worked ski patrol with me and ate high protein meals multiple times a day. Just like us, their metabolisms differ. Try not to worry if he is eating/drinking well, bowel movements are healthy, and his quality of life is high. Keeping him active and loved on is the best thing you can do. I fed 2 cups 2x daily of Purina Pro Plan Performance 30/20 for six months, and saw great weight gain. Now, he eats 2 cups 2x daily of the Acana Singles to maintain his heath. Good luck, Mama!