Wowzer, that's some really scary stuff
Yes, getting rid of the sugars means getting rid of his grain altogether.
That being said, if you're working him hard enough, he may need some grain added back in.
That's something you will have to monitor and maybe even contact an equine nutritionist. I know MTSU in Murfreesboro, TN has one for example. You could check with nearby universities that have equine programs.
A ration balancer would be grain-free yet provide him with his vitamins/minerals as long as it's fed according to the directions.
Beet pulp has already been recommended for keeping weight on and that does work.
I stand in the minority of horse owners who has no use for beet pulp. I prefer equine rice bran. It is 22% healthy fat and provides cool energy.
If you were to buy rice bran, make sure to buy equine rice bran that is stablized and calcium fortified.
I hear of horses colicking all the time on beet pulp, regardless of how it's prepared. I've yet to hear of a horse colicking on rice bran.
Even though I get taken to the woodshed for it, I feed rice bran to my two insulin resistant horses They do fine on it - they get blood tests twice a year that tell me that but that doesn't mean all metabolic horses could tolerate rice bran.
If you can get EquiJewel, if you're interested, it's a great product. It has to be fed with a vit/min supplement of some sort; either a ration balancer, bagged feed, or just a plain old ration of vitamins & minerals.
I love EquiJewel but can't get it anymore so I buy Manna Pro's pelleted rice bran.
If you start feeding hay cubes, soak them in scalding hot water at least 15 minutes ahead of time. Mercy those things are rock hard - lol lol
One of my mid-20's horse's has four molars missing and I have to mush those cubes clear down to wet grass before he can have them.
Scalding hot water breaks them down pretty fast and by the time you get them to the barn they're nice and warm to put in the feed pan.
I have been feeding Standlees from Tractor Supply since last winter and have been really happy with them