Wild Spot hit alot of the points I wanted to make.
A horse with a history of colic and seems to be sensitive to corn-based feeds would point towards possible gastric ulcers. Even the drinking less in the winter can be because of discomfort caused by cold water when ulcers are present. You should really discuss this possibility with your vet. If GI ulcers are present, then you need to treat them.
But even before your vet checks or treats for ulcers you should make some feeding changes. Strategy is a grain based feed (corn is a grain and one of the highest in starch) and grains (high starch feedstuffs) tend to trigger painful episodes in a horse with GI ulcers. Grains also increase the risk of GI ulcers. It would be best to remove grains from his diet altogether. Since purina feeds are available, I would recommend going with either Enrich 12 or Enrich 32 which are both forage based and designed to balance the nutrition that a horse receives from hay. The provide free-choice hay or pasture. Also provide water that is tepid rather than cold or warm.
All of these recommendations are actually applicable to any horse to help prevent colic.
Here are some good articles for you: http://www.thehorse.com/ViewArticle.aspx?ID=12957 http://www.thehorse.com/ViewArticle.aspx?ID=5337 http://www.thehorse.com/ViewArticle.aspx?ID=10133 http://www.thehorse.com/ViewArticle.aspx?ID=10551