Have you tried more hay? My mare will leave hay b/c she doesn't want it, I have mixed strategy and grain (just oats, grain and molasses) or alfalfa cubes and that gets her to eat it b/c its different ... picky I know .. good luck :)
Fat is better to gain weight than low stratch unless the horse has a problem digesting the feeds (insulin resistance/cushings, etc.). If the horse is a hard keeper, then you'd be better off trying something like a stabilized rice bran pelleted feed with a high fat content. If the horse has insulin issues, then you can try the low startch otherwise it won't put weight on him.
The Low Starch is meant to be fed in high quantities. If your hard keeper isn't keeping his weight on the max amount, THEN I would add alfalfa pellets and a fat source, or switch to another feed.
I believe you can feed up to 3% of your horse's body weight a day of this feed, which would be 30 lbs for a 1,000 lb horse. Your horse shouldn't need any hay or pasture.
Personally, I don't like feeding high quantities of feed, or really any feed. If you can get TC, I would feed your horse free choice grass hay and their 30% supplement. Give it a month to see how he does on it. If he loses, then add oil or rice bran and whole oats or alfalfa pellets.