I totally feel for you! My horse is a horrible I finally have his feed down at the right wetness that he hasn't choked in a couple weeks. If it soaks too long he chokes or not long enough, it is horrible, but he has a thyroid tumor which we think is why he chokes. Thank goodness I work at my vet, and the day he was choking really bad the vet was there.
Pretty much what everyone is saying is what my vet told me to do. I wish they had feeders like they make for dogs that eat too fast. I tried to make one myself but my horse got mad and dumped his food over. I'm glad your horse is doing good, and hope it doesn't happen again.