Does this mean he is in all night with no hay? If so, that would be a bigger problem than runny poop.
If the pasture is that rich, you might do better giving him a flake of hay instead of grain. If he IS in all night, more than a flake would be advisable.
^^^^Ditto that and shame on the barn owner if that really is the case.
Horses are forage animals - they have been since their hooves hit the ground thousands of years ago. It's a wonder your horse hasn't colicked by now.
If he is in all night, he needs to be 1.5% - 2% of his desired weight in hay and that information can be found on any credible equine website, including university sites.
I have heard of some questionable barn owners that barely give a horse hay because a bale of hay cuts into their profit margin.
I think this is the first time I have ever heard of a barn owner not giving a horse any hay when it's locked in a stall all night. The BO should know better -- is this a non-experienced family or friend doing you a favor? Regardless of who it is, if all your horses come in at night, they have to have hay.
Yet they will feed grain? You must be providing the grain?
The acidic juices in a horse's stomach never stop trying to process something which is why horses pretty much graze 24/7.
Grain at night and no hay for the entire night is also a good way to encourage ulcers and colic.
The loose manure stands a good chance of righting itself, if the horse receives hay at night. If not, giving the horse a quality probiotic might help. Probios is not expensive and can be bought at Tractor Supply.