You'd need to work him up to being out on pasture 24/7. Start out with a couple of hours in the morning and afternoon then gradually increase the time an hour or so a day until he's out there all the time. Even then, it's usually not a problem unless you have extremely lush pasture or pasture that has alfalfa or vetch or clover in it or if it's early spring and the grass has put up really fast. My sister had a horse colic on vetch in one afternoon once but it was belly deep and the horse had basically been on a dry lot before then. It was her first horse and neither of us knew any better at the time but we did after that.
Another big and important problem with a horse suddenly being turned out to pasture is not just colic, but founder too, especially if the grass is rich.