To me, if he had no prior issues that says that something about your current situation isn’t jiving with your horse - whether it’s the environment, the hay type, daily routine, training style, etc…It’s wild how much of an impact these things have on horses health and digestive systems.
In my situation, my mare had no prior issues either until she was brought to the barn I kept her at those first couple years. And they immediately almost entirely went away after I moved her out of it, even though it was a fantastic barn with good care. She hated turnout there and I often had to bring her in after only 15 minutes, while day 1 at the new barn she was out for 8+ hours and loved every second of it.
If his diet initially remained the same with you as the barn he came from, that could point to something in his environment as well. Lots of truly excellent boarding barns with great care just aren’t right for certain horses - some thrive in a busier environment, while others prefer peace and quiet. In my mare’s case, she was the latter. She needs as much overall “peace and quiet” as possible and any excessive busy-ness, change, or hustle and bustle causes stress to her that ultimately impacts her GI system. It was a huge sacrifice for me as I had to move her an hour away at the time, and ultimately moved my family to a new town a few months later so I could be closer.
Not suggesting you have to move to a new situation, but it’s definitely a factor to consider when looking at the big picture 
Happy to hear you’ve had 3 days colic-free…I totally understand the feeling of what a win that is!
