A daily electrolyte supplement might help if the colic is stemming from not staying hydrated. Being well hydrated will help keep the stool soft if he is impacting when he colics. You might also try giving him oil as well to help soften stools, similar in theory to when the vet pumps mineral oil into their intestines to help things along, the daily oil is supposed to help keep things moving and help soften the manure being passed. Bonus it will make his coat nice and shiny too.
Try Ulcergard. Do a loading dose say 2 full tubes in a row and then 2-4 more using the standard directions of 1 dose per day( 1 tube = 4 doses) til finished then at least one tube per month or if you feel they don't seem right then give a tube ( each tube will be for 4 days straight except the first 2 which will be a full tube).
How many horses on the property? I've had one with chronic colic when he was out alone, or with to many others. Magic number was 4 lol. Took a long time to figure out the issue. Almost had to euthanize many times.
Maybe ulcers are causing his problems?
I know a horse also that gets frequent colic for him it's gas issues. When he gets colic we use a photonic light(I hope that's how you spell it) it has saved him many times.
I'd say the only thing you can do is keep a close eye on him and banamine nearby for severe cases so you can administer it while the vet is on their way if they think nesicary. Aside from that you seem to be doing everything preventative that you can.