I don't think your main problem is feed, maybe training, maybe just a personality clash, but you asked about feed so that's what i'll talk about.
I have recently (In the last year) switched from feeding a sweet feed to feeding no grain, low starch and low sugar diets to all my horses.
Why? One of my horses is grain intolerant and has chronic tying-up issues when on grain or sweet feed.
I am now a complete convert. Low starch and low sugar diets are healthier, more natural, and don't have to lack anything contrary to popular beleif. You can provide as much energy and goodness as you can with grains - But without the bad bits.
Sweet feed to horses is like junk food to people. Just not good. A horses digestive system isn't equipped to handle the massive spikes in blood sugar and starch that they get from a seewt feed.
So - alternatives.
Is he turned out? For how long? - If he isn't at 24/7 pasture, then try and get free choice hay in front of him 24/7 when he isn't grazing. 24/7 forage is what horses are meant to be eating.
Next, add a mineral/vit supplement or ration balancer. These are designed to complement the forage based diet and 'top up' any deficiencies they may have. My horses have a vit/min lick in the paddock. A ration balancer is a pellet that is fed at very low rates.
Next, you add calories as needed.
I personally love, love, love beet pulp - Called speedi-beet in Australia. It is classed as forage but has roughly equivalent energy levels to oats. It is usually fed soaked so it great to hide any unsavoury additives.
Calories - Many different options. You can go a grain substitute like lupins, flaxseed meal, soybean hulls. You can try oils. You can try 'meals' - Copra (Coconut meal) Rice bran, full fat soy, etc.
Personally I feed a base of speedi-beet and add copra for calories.
Or you can try some of the low starch, low sugar complete feeds that are out there. Most are based on rice bran, FFS, soybean meal, copra, or beet pulp, with vit/min added. I've tried Prydes Easi-sport and Hygain Zero - Not sure what you have.