If it was too much for his body to handle, he will colic. But there's really nothing you can do now to prevent it.
Watch for signs of rolling (a lot) and pawing/biting at his belly. If they seem intense, call a vet out.
What kind of dewormer was it? Brand?
He should be fine honey.
Horses do this kind of stuff all the time.
Watch him like a hawk, make sure he is still passing = pooping. Is he showing any signs of discomfort, rolling, laying down, hanging head, groaning, if so get a vet out asap!
I don't know what kind of wormer he got into, granual? There are dosing instructions and you should be able to tell if he's had too much... If he ate a tube of paste he should be fine. Never seen a horse do that!
You know, I think I would give him 2 Tablespoons of regular table salt in a handful of something you know he would eat (just enough to get him to take the salt, assuming he won't just lick it out of your hand). Then, keep water buckets full around the clock so that when he gets thirsty he'll drink, drink, drink. The best thing you can do if he is headed toward a possible colic is to make sure he's super well hydrated. I would give him 2 T each day until you know the danger of colic is past.
If I thought he ate a whole bag of anything, I'd go ahead and get a vet out! The sooner you catch something like this the better, and you don't want to wake up in the morning and find your horse has been down for hours with a serious colic.
Lt was right, LOTS of water to keep him hydrated and his bowel moving.