I had a horse that would always be harder to keep in good condition in summer - his situation wasn’t that he was eating or drinking less, but he was moving more in relation to flies/bug harassment. Even fly-protective clothing/measures didn’t help. Basically, he was burning more calories in the hot weather because not only did we maintain our standard work, but he was moving more independently. (And I’ve heard it said that you burn more calories in hot weather because your body works harder to cool down - not sure if this is true or not?)
We ended up adding a ration balancer to his diet year-round and the amount of it went up in summer. His original grain was pro force fuel (a Nutrena feed), and we actually reduced the grain (he was getting about 3lbs 2x a day) and added the ration balancer (also a pelleted feed) to it, and that helped him keep in good condition even in really hot, swampy summers that he’d always struggled in prior. (A side note: I didn’t need to feed the volume of ration balancer suggested by the label - I only fed about half a pound 1xdaily, though we did increase it some in the worst weather.)
And my horse enjoyed yellow gatorade being added to his water, although you can see which “flavor” they like by seeing if they’ll lick it off your hand (blue is the other flavor I’ve seen used pretty frequently).