A stallion that I had would stock up for a variety of reasons, but when worked regularly and still stocked – it was usually due to the feed. At the time he was fed alfalfa and all-in-one. When I reduced alfalfa and added either grass hay or oat hay, our problems diminished quite a bit – he was an easy keeper, so calorie / protein intake was never an issue. You noted that your horse is turned out – is he on grass? If it is Bermuda, and this happens especially in October, Bermuda has a tendency to spike nutrient levels just prior to the first freeze that causes it to go dormant. During this time (in California) I reduce my horses turn out due to a variety of reasons. Also, I found that after a work (when he was stocking up), if I wrapped his hind legs for a few hours, it would also reduce the issue.
I hope this helps!