Good ol' Gramma Pony has eaten cat food, dog food, bird food (seed mix as well as pellets formulated for breeding finches), people food, uhhmmm...that's just what I KNOW about.
My vet says in moderate amounts it's unlikely to HURT her or any horse, however it's not a good idea for a longterm diet to have anything other than food formulated for horses/horse appropriate.
I am sure your guy will be fine, from the sounds of it.
My Arabian once ate a half a bag of cat food thanks to some neighbor kids. She was fine once we got her put up away from the helpful kids, though her poop DID stink something awful. My vet had me feed her just hay for a day or so pending what happened. It was a big bag, like 20 lb.
I've known of a lot of big horses who've gorged on dog or cat food and have lived to tell the tale with no problems, but it' s a good idea to keep an eye on them anyway. Good luck...
Liz M.