I've always been told not to feed horses corn since it's a "hot feed" and not worth much nutritionally.
Depending on what the BO feeds, if there are other grains mixed in with the corn, the quality of the hay fed, and needs of the horse, you may want to talk to the BO about only feeding your horse hay. If the hay isn't that great, or your horse needs extra feed b/c of special needs or activity level you may look at adding a ration balancer or supplement.
Just my two cents! :)
Edit: sorry - just saw you said "his horse" not your horse! All in all, I wouldn't feed corn if it were me!