This may not be full blown narcolepsy, he may just fall asleep too deeply when standing. It could take a few minutes for him to go from standing rest to the deep sleep causing him to buckle his knees. Most horses with sleep disorders, only have occurring episodes during normal standing rest.
I would definitely recommend seeing a Vet, because this isn’t considered normal behavior.
I hope it is as simple as taking him off or reducing his supplements! Sending jingles for you and your boy!
I actually very recently spoke about this with my vet. She mentioned that some horses (especially an alpha horse but really may be any horse) when they change their routine (day versus night turnout, different pasture buddies, or in your horse’s case permanent field board versus having stall time to himself) they will tire themselves out to the point of passing out in cross ties. This is because they never take a break from keeping watch over their herd in the field. Your horse may appreciate some stall time to himself to catch some zzzzz
You can just leave him in for few hours after he eats his am meal. That way he isn’t in for that long and will get out of the heat for a little while. I catch my horse laying down sleeping every morning after he has eaten his breakfast. He is turned out all night.
This exact same things happens to my boy. It’s probably sleep deprivation. Many times horses who don’t get enough turnout or who are the pasture leaders/alpha horses won’t get enough REM sleep because they don’t lay down for whatever reason. This causes them to randomly fall asleep like this. Unfortunately, I can’t offer a solution 
My guy goes out all day but I can’t change that he is the pasture leader. I just try to monitor it. Some days are worse than others. I talked to my vet about it and he didn’t seemed too worried. I don’t think it’s Narcolepsy just because it seems that in your case and mine, the horse only falls asleep like that when he’s already been dozing for a while. It’s not sudden or without warning.
Here is a very good article on the subject.
