I would give him half a bale in the morning and half at night. See how he does on that. If he has a lot left over when you feed, reduce it to 1/3rd bale.
When you put the hay out, spread it to multiple piles. This helps keep him moving and slows him down, so he doesn't scarf it down all at once.
Also, hay digests "hotter" (internal heat, to keep the horse warm) than grain, so if you're feeding any grain or horse feed and you notice him gaining weight, it's better to cut back on the feed and keep the hay. For this reason, I feed my horses a broad spectrum vitamin/mineral supplement along with alfalfa hay pellets, no feed or grains.