Hello. Thank you for asking your question on just answer. My name isXXXXX will try to help.
The two most common problems with eating moldy hay are cough and colic.
That is a pretty fast reaction if he just ate it.
Colic is abdominal pain. The signs are tail swishing, looking at the sides, pawing, rolling, going up and down, not eating or drinking, grinding teeth, and rapid breathing. Laminitis, our founder, is inflammation of the sensitive part of the hoof. If you look at your finger nail, the sensitive part is the part under the finger nail. That is like the lamina in the hoof. When there are bad toxins in the blood stream the blood supply to the hoof can be disrupted. This causes the lamina in the hoof to become inflamed and painful. The horses will shift their weight from one foot to another over and over. They also may stand in a "parked out" stance where the front feet are stuck out way in front of them. The hooves may get hot as well. If the laminitis gets severe, the coffin bone inside the hoof can start to move and can even sink out the bottom of the hoof.
If he continues to refuse to eat, or shows any signs of colic that I listed above, then you should have the vet out.
If you can not get a vet out, give banamine every 12 hours for the next 24 hours. This is for abdominal. The dose directions are on the tube or 1,000 pound horse gets 10cc in the muscle unless you are an expert with iv injections do not do iv.
You can go to the drug store and buy mineral oil. Give 1,000 pound horse about a gallon ,by mouth, with a syringe. Point the syringe towards the cheek and give it slow. You do not want to shoot any down into the lungs or you will cause pneumonia with the oil.
Mold spores can get into the lungs. If he starts to cough, then he will need to be checked by a vet and may need to be put on an anti fungal medication.
Many horses eat a little moldy hay and do just fine. Hopefully this will be the case for your guy.