I feed all my lactating mares, foals, weanlings until they're 2 on Purina Mare and Foal and Ultium Growth. It has everything they need except salt and is formulated for controlled growth. The Ultium is higher in fat content and really helps the bloom. If you haven't already, I'd deworm your little guy with Ivermectin, I also do that once a month until they are 1 year old. It really helps to keep that pot belly down and to keep them thrifty and growing well. Dr. Deb Bennett says a foals ribs should show for the first 2 years of their lives, I don't know if I totally agree with that but you should be able to feel them. Her concern is that by over feeding we're creating various joint issues like DJD and OCD in the youngsters.