As a general rule, a horse needs 2 to 2.2 pounds of feed for every 100 pounds of body weight. (You can buy a weight tape to measure how much your horse weighs.) For example, an average 1000 lb horse would need 20 to 25 pounds of feed a day. Most of that should be hay. A typical diet for a horse being ridden for one hour five days a week would be 2 to 5 pounds of grain and 15 to 20 pounds of hay a day, split into at least two separate meals. On a final note... It sounds like your horse needs to be wormed. Let us know how things work out afterwards.