You can increase a horse's BCS by a point in 30 days by feeding more grain and hay. But body condition isn't the only consideration now. Your horse also should be on a scheduled deworming program. Horses need to be dewormed every other month with ivermectin, every 90 days with moxidectin or every month with other dewormers. Although you probably vaccinate your horse in the spring, make sure your horse has been immunized against tetanus.
Horses often get undetected cuts that are ideal for the tetanus microorganism.
Remove shoes since you won't be riding your horse in the winter. Loose shoes can cause serious injuries. Make sure the horse's hooves are properly trimmed. And don't forget to have them trimmed periodically during winter. Don't turn horses out, even for a short time, with a halter on.