Is he the only horse out there? Does the BO not have her own horses that need feeding?
Horses should be fed hay at least twice a day. If you cannot do this, then a round bale is your best option. You can always get a grazing muzzle to limit his intake.
Or, if the pasture is large enough, put hay out in 6-8 different places spread all over, at least 30 ft apart, so he has to walk between piles when eating. This will slow him down and keep him "grazing" longer.
If your barn isn't too far away, get up extra early and feed your horse before school.
If none of this works, then you need to find your horse new living arrangements.