I helped take care( as a volunteer) of a few Botulism cases at New Bolton. There is a medicine for it, it is expensive and certainly way too expensive for a full size horse. But it may be a bit more reasonable for a mini, maybe.
If he is down and laying down. It is very important to turn him from side to side many times a day. Also, very important is his position while laying down. Best to be NOT FLat on his sides. The Best position is laying .... like a dog, on his stomach with front feet out in front, or at least have him propped upright, so that he is more up than flat. This can be done by positioning him with pillows or even blaes of hay. It is important to turn and move him to prevent pneumoina and to allow for "drainage" of the sinuses, airway and lungs.
As far as I know, it is not so much moldy hay that causes this as it is usually caused by.... say.... a dead snake or other creature that goy caught up when the hay was baled and the creature decomposes. The animal eats the contaminated hay. Thats why I won't feed round bales, as when I used the square lady bales, I can see what is in the flakes of hay that I am feedings, but still there is no gaurentee.
I wish you the best of luck, sending prayers.
Robin