I am just glad that all of you came out of our mothers womb knowing all of the answers to all of lifes questions. Thank GOD for that!
I "got" (at least I think I did) what Speed Racer was trying to say.
I can be taken so far off the mark that somebody on another forum accused of having a "nasty little attitude"
Horse management is a lot more complicated than when I started being responsible for my own horses 54 years ago. Back then my horses ran 24/7 on 100 acres and got called to the barn twice a day with a handful of home grown oats as a reward. Mine was an old cow barn with a big open ex-machine shed attached. Meaning they came and went as they pleased and nobody got sick on grass --- ever.
Fast forward to today's world where I am now fully retired and my second group of Keeper Horses roam on 22 acres of native grass on steep hills. They can't go anywhere to graze without walking up and down hills.
Wouldn't ya think that, alone, would keep them healthy?
One horse became metabolic in 2007 - luckily he has never had even a mild laminitis attack. I put all four horses on a low starch metabolic diet at that time.
Lo n Behold, a second horse was formally diagnosed with IR in 2010. He had light bouts of laminitis every Spring and finally went into a grazing muzzle for 9 months out of the year.
For all my caution, he still grass foundered in the first week of March, 2012.
He's coming around nicely but my point is, I have 54 years of serious horse-keeping experience. I did everything I could think of, at the time, to keep these horses healthy and I still had one founder on very early Spring grass.
You did the right thing by building up your horse's grazing time but it sounds as if your horse is predisposed to metabolic issues. You probably could have wired his jaw shut and he still would have found a way to founder, as my thought is he has become that sensitive thru the years.
Metabolic issues are the same as Type II diabetes in humans. Some people need to be on meds as children, others don't develop the disease until a later point in adulthood.
If your horse can graze at night and be put up in the daytime, that would be best.
If the boarding barn can't do that, hopefully they are willing to put a grazing muzzle on him daily and take it off when he comes in at night. If he doesn't come in at night, the muzzle still needs to come off at dusk as they are not meant to be left on 24/7.
Someone also needs to make sure he is drinking water thru the muzzle. My IR horse went on strike for three solid WEEKS. I had to bring him in every afternoon and lock him up with hay, water, and WHITE salt until he figured out the muzzle wasn't going away.
Also, if you look thru the threads in "Hoof Care", you will see a few current threads discussing this very issue. There might be some more helpful information for you
I hope this helps some