Until last year, I can count on one hand the times I've dealt with colic in 54 years
Until last year, when my heart horse of 22 years colicked NINE times between May and October. Serious-have-the-vet-out-tube-him-the-first-two-times colic
He tested positive for hind gut ulcers. He also has equine metabolic syndrome so his diet is "lite" to begin with. I had to change his diet yet again and started him on Succeed which is a prebiotic.
Yes, as Speedracer commented, severe swings in temps can induce colic.
If your vet is suspicious of this being sand colic, you might ask what he/she thinks of starting this horse on a daily feed-thru of psyllium. Equi Aid Natural Psyllium Fiber Supplements
I fed equi-aid, daily, the entire five years I had my horses in the Low Desert of Southern California. None of them experienced colic.
You might also ask about the possibility of gastric stomach ulcers or hind gut ulcers, as either one will cause colic.
Even if you don't think your horse is under stress, it could be and that could be a reason for the colics (and possibly ulcers).
My Arab is so laid back, we call him The Stoner Horse, yet he has gastric ulcers. I was surprised when the vet said he wasn't because this horse is the Passive Leader (2nd in command) and acts as nursemaid whenever I have a sick or injured horse.
At any rate, repetitive colics are an indication that something else is wrong. The vet should do whatever tests he/she feels appropriate to start the process of elimination. Once diagnosis is made, then you will know what to supplement with to hopefully avoid future colics.
I did all that with my heart horse the first of October, last year and <knock-on-wood>, he hasn't colicked since and we have experienced some really wild weather swings since last October.
Many Kudos to you for being such a savvy horse owner and best of luck getting this figured out