Is this to much food for my horse?

I have a 18 hand Thoroughbred stallion who I rescued about a month ago and he is very under weight, he's probably a 4 on the body scale. For the past month i have been feeding him 2 flakes of gr hay in the morning and 2 flakes at night and then 3 quarts of Purina Senior Feed morning and night. But he's not really gaining weight. So now im thinking about doing; 3 flakes of hay in the morning and 2 quarts of Purina Senior, 1/4 cup of corn oil, and 1/4 cup of rice bran. Then at night I'd give him 2 flakes of hay and just 2 quarts senior food but with NO oil or bran. Is this to much for him? Please let me know. Thank you and God Bless.