There are a lot of variables in treating horses with insulin resistance, which your horse sounds like it has.
Have you ever had blood work done to determine insulin and glucose levels?
Were the hooves X-rayed to determine degree of rotation?
When Get Smarty went thru the electric fence, was it working? If so he's a tough one, you need to buy a cow fence charger, and make sure the dirt around the grounding pole is soaking wet
He needs to learn to wear a grazing muzzle. The best ones I found for my horses are these Saddles Tack Horse Supplies - ChickSaddlery.com Tough-1 Easy Breathe Grazing Muzzle
Notice the big nostril holes. Not being able to breathe is a lot of why many horses pull the muzzles off. The grazing hole is a little bigger on these but it saves the teeth because horses are great at chewing a small hole bigger
I have two insulin resistant horses. One has to wear his grazing muzzle underneath his fly mask or off it comes. He not only knows how to get it off, he knows WHERE to lose it ---- waaaay over on the property line, across the fence in the neighbor's hay field
Regarding diet. No grain whatsoever. And for this horse it sounds like no soy either. The protein source in 99.91% of equine feeds and vit/min supplements is soy.
He's better off with a ration balancer instead, along with his hay.
Can you bring him in at night? Muzzle him for daytime turnout (unless the temps are freezing or lower), then bring him in at night, weigh his hay (literally), pack it into a slow feeder hay net that you can also buy from Chick's saddlery. Saddles Tack Horse Supplies - ChickSaddlery.com Slow Feed Small Mesh Hay Bag <>
I have one of these for the one IR horse that thinks he needs to eat himself to death before dawn
I weigh his hay, put the bulk of it in this hay bag and put some "free" hay loose in his hay tub. I tie this net securely over the hay tub. He is barefoot so he can't get caught if he were to start pawing it.
Horses are supposed to have 2% of their desired body weight in hay. I don't live by that rule in the winter. My fella is 15.3H and weighs 1,100 lbs. He comes in around 6:00PM every night and doesn't see me again until 6:30 the next morning. I give him 5 lbs of hay and he always has something left over so the slow feeder net is doing it's job.
I know others will come in with suggestions and I'm sure I haven't covered everything.
The bottom line to your horse is that it sounds as if it's at risk to founder again because, yes, that short period of time on grass can do that much damage to extremely sensitive horses. Your horse sounds as if it falls in that category but there's a lot of missing information
I hope this helps a little