This is all from information I've read or discussed with my vet over the years.
Adding to what @tinyliny
said about this possibly not being a full on case of colic, if the vet did not specifically say that there was an impaction, then it could have been irritation. My mare had that one time - laying down, not feeling good, looks like colic. There's an easy way to test if your horse has a gut full of dirt or sand: get a clear bag (ziploc works) and scoop at least 5 manure balls into the bag. Fill bag 3/4 full with water and shake it up. Let this rest for an hour. Is there a bunch of grit at the bottom of the bag? If so, share this with your vet. Your horse might need a psyllium treatment, but ONLY if the cause of discomfort is sand/dirt in the gut. You do not want psyllium to cause an impaction or make an impaction worse.
It's a good idea to keep a notebook and pen with you so that you can remember and share information with your vet. Jot down things like your horse's behaviors, anything strange or new, when he poops, poop consistency (well-formed balls vs diarrhea), gut sounds and how often you hear them, breathes per minute, heartbeats per minute, and temperature. If you're not familiar with taking vitals, here's a good guide with a video: https://www.smartpakequine.com/conte...es-vital-signs
Do you have a stethoscope? Sometimes drugstores or local horse-oriented shops will have them. You can use it to listen for gut noises, or just rest one ear against his lower abdomen while covering the other ear. Roaring, rushing, growling, water-trickling noises ... all good signs. Gut silence is not typically a good sign.
If your horse is in a dirt paddock and steadily loafing around every hour, keep him there. If he's doing anything more than an occasional lazy trot, he needs a smaller enclosure. Keep him standing up as much as possible, and make sure he's walking around a little bit...about every hour should suffice. You can hand walk him in straight lines or large circles in short (~5 minute) intervals. Go easy on him - no tight turns, no quick movements.
If it's cold out, keep him warm with a blanket and/or indoor stall time. Also adding to Tinyliny's post, there's a trick that every horse owner can safely administer to encourage water drinking, including when it's cold out. You can sprinkle about 1 teaspoon of table salt (iodized is fine) over any supplements that a horse gets. Even if you give a salt brick, you can't guarantee that your horse will lick the salt brick. Most horses don't get enough salt in the diet, so a moderate amount of salt is fine. Just not too much at one time.
And even if you don't think you need to, it is always favorable to add water to any dry supplemental feed or grain. Slime factor helps prevent colic situations. Take the total amount of "dry supplements" you feed and add 1/4 to 1/2 of that amount in water. How much you add depends on whether your supplement/grain will absorb the water or not - more if the supplement absorbs lots of water, less if it becomes soup rather quickly. Mushy is okay, 'cookie dough' consistency is okay, even thick oatmeal consistency is okay. Some horses are picky and won't eat it like that. Sometimes my mare won't eat it unless I feed it by hand, but most commonly she'll eat it if I put salt in it or feed it in a shallow, wide dish. You can also try adding a tablespoon of Uckele Cocosun as a topping - I've been told that stuff works like a charm.
Remember that there are some feeds that NEED to be soaked or you're asking for trouble. For example, beet pulp always needs to be soaked overnight - 1 part beet pulp to 4 parts water. Hope all of this information helps you.