IF possible, just let him start out grazing for a couple of hours at a time, in the early morning or at dusk. Work up to overnight, because night is better (less sugars that cause laminitis). If he's an easy keeper, you probably don't want him on lush grass 24/7 and whatever you do, try to keep it consistent, or about the same amount of time each day, not feast one day, then starve the next, or that will be apt to cause all sorts of digestive upset.
It's common to see them have the runs after any change in diet, but the main thing is to take is slow with the changes.
Acute Laminitis occurs about 72 hours after the trigger, but if you suspect your horse was overdosed on grain or grass, get him moving before that 72 hour window is closed, and he's apt to metablolize some of the sugar and prevent, or at least mininmize the pain. Once the pain is set in, your horse should not be forced to move around, for another 72 hours. After that, a good trim and walking are the best things for him, aside from not overdosing him on the same feed (but don't starve him, or you aren't doing him any favors, either).