Bolting

We have a former ProRodeo 1D barrel horse that due to his last owner leaving him without care or being ridden, has become very skinny . He gets a scoop and a half of grain everyday twice a day but he drops so much grain that he seems to only be eating 3/4 of his scoop and a half . We're planning on having his teeth checked very soon but is there anything we can do beyond that ?
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Until you have his teeth checked, can you water down his feed and make it into a mush so it's easier for him to eat?

I like to use hot water, because it absorbs faster.