How Can I Help My Rescued 4-Year Old Toy Poodle Build Muscle Mass? He Previously Spent Most Of His Life In A Crate And Was Mostly Inactive. He Eats Well And Exercises And Is Free To Roam Now.

Recently taken in a 4 year male toy poodle who unfortunately spent 98% of his life in a crate. At first I thought he was skinny however I have verified that he was fed well just had very little exercise. It appears to be more that he has NO muscle on his body. You can feel his bones including hips and not feel any muscle. What is the best thing for me to do to get him into good shape? How can I help him to build muscle? He eats well and runs around all day with my other dogs