I would up him to a cup of it right off the bat, it's a fiber, not a concentrate, so 1/4 cup really isn't going to do anything for him. It's much more likely that he'll need around 2 cups, but a cup is a good place to start to get him used to it.
There are lots of good feeds. The diet I use for the show horses is a mix of Enrich 32(a multi-vitamin, 32% protein supplement) and whole oats. The mix I use varies per horse but it's usually anywhere between 1/2 to a full cup of enrich 32, and 1 to 2 cups of oats. If a horse really needs weight, I will add to that a sweet feed like omolene 300.
Before that, I only used omolene 300 and whole oats, but not every horse needs the sweet feed, and I preferred the higher protein in the Enrich 32. The higher protein did a good job of putting weight on the topline as well.
Depending on who you ask, you'll get lots of recommendations for "good" feed. Those are just two ways I've done it. Our horses were on TC Low Starch way back when and it was awful. The farm we bought the horses from used it because she had several horses with insulin resistance problems, and for those horses, the feed served it's purpose, however after a while we realized it wasn't doing our show horses any justice. After that we used the platform mini feed. That was an equally poor choice. Several people will say that it was great for their "easy keepers" but it didn't work for us.
It will also depend on your area. In some areas it is harder to get certain feeds.