To Riding For Christ : refer to my comments above. Purina Strategy and Manna Pro Grow N Win were recommended by my veterinary for weight gain.
Strategy GX is a safe feed, so feed maximum allowable amount per horses weight. Add one pound of Grow n Win on top of Strategy two times a day. Strategy will not make a horse hot or excitable.
If you can have good quality brome hay, or grass hay for your horse all day long this is beneficial in many ways.
You can also try Cool Calories supplement. It does not make a horse hot either. It is a sweet powdery substance used once a day.