If the grain worked in the past, then give your horse grain. Dont let your horse get too skinny just because your scared of getting him hyper, always do whats best for your horse. I have taken a look at another question of yours, a d it looks like your having problems with your current trainer. It may be time to switch, although I always strongly encourage to make sure your absolutely certain 100% your making the right choice for the right reasons
I use some petunia feed for my horse and he can get high stung very easily. Just start him out slow so his body can get used to it. I'm not English but I'm always happy to help anther horseman
Find a nice grain that'll keep him calm. Just give him some rice bran (it's pricey but worth it) and that should do the trick!
The purina impact grain is also nice because it provides a lot of nutritional value with out over energizing the horse. It's really nice 10/10 would recommend!
Grain is a very important aspect of a horses diet because it provides nutrients that it can't necessarily receive from just hay. If it's the horse getting hot that you're worried about, Purina Impact is a wonderful grain that is given to my horse pelleted or sweet it doesn't matter. See if just putting him back on grain helps. But be gradual with it as dramatic changes in diet can make them ill. And gradually decrease any large amounts of hay without illuminating it entirely because hay is also necessary for nutrition. You want to try to balance the two out. You can also look into changing your hay to alfalfa along with the grain if just the grain doesn't work or some weight gain supplements.
Just give him some high fat. But not high energy