Are you giving extra salt in their grain daily? Horses worked and sweated, need more salt than they can lick off a block. Feed salt, plain, available at TSC, is what we give our horses. A Tablespoon, once a day to the unworked but drinking OK. Two Tablespoons once a day to the workers or poor drinkers. They seem to eat grain just fine with the extra salt. We only grain once a day.
You CAN pull out his tongue, pour on the salt to get it in them if needed. Had one poor winter water drinker. I told her she WAS going to get the salt in her one way or the other! She did not care to have her tongue pulled out, salt applied, so she started eating the salty grain and wet beet pulp!
Something to consider, is horse usually getting most of his water from a tank? He “may not lIke the flavor” of buckets, containers home water comes in. Not be used to drinking from buckets much. We quit bleaching or using soap to scrub stall buckets, horses refused to drink until taste-flavor-smell was gone in about a week! They drank plenty from the tank outside during that week!! You may want to dump or remove the water tank, hang a row of buckets and fill them daily until horses are used to bucket water, and empty them.
Salt will help them retain water, as well as making them thirstier. They NEED to be accepting of various watering containers for travel. Not drinking at a show is frightening! Had a friend with a horse like that. Got to the show, no drinks in the trailer, won’t drink in new stall. Now at about 18 hours without water. She threw up her hands, loaded him and went back home, 12 hours each way. We use the rubber feed pans to water in the trailer manger, bucket is too tall to get in the window. Ours have had water in a variety of pasture containers, small rubber tubs, new muck buckets, plain buckets, no shy ones here. They KNOW trailer ride might be long, so they drink well when water is offered. Young horses watch older horses, copy them getting drinks.
Never did the feed in water, but do use kool-ade for flavor, which they have drank at home before we left. Good for sulphur water smell.