The more they drink the better in my experience. They might also have a good sensitivity to certain things (like soy) or struggle with long stem grass, in which case you'd need to feed chopped hay. You need to figure out what the CAUSE is. It's a process of elimination.
Don't let a horse drink deeply after a long, a hard ride, or even just galloping through a field.
I agree with Emma. Try the plans that smartpack has. Also, keep the horses feed, water, and sorounding the same. Don't make any sudden changes in his lifestyle.
Hydration!!! SmartPak has a colicare plan I think that probably comes with more tips too☺.