Hi, firstly, I'd want to find out why he's been such a 'hard keeper' & endeavor to get his gut healthier. Unless you're doing long/hard riding for much of those 2-3 days, I wouldn't consider him in moderate work, so he 'should' be a good weight on a maintenance/light work ration, if healthy. Obviously teeth & worms are possible factors to rule out first. If he's been known for colic - and laminitis may well have been directly associated too - there have very likely been ongoing feed/management problems, commonly too rich/starchy feed such as grain. I'd consider feeding something that promotes gut health & healing, like Slippery Elm, Aloe Vera juice or such.
Maybe what you're feeding is great for him, and likely better than his previous owner did, by the sounds. But I'd want to find a low starch/sugar alternative, especially if his digestive system is struggling - only know about the feed from the product info page, doesn't give NSC values, but it is grain based. You don't say how much or how often he's fed this. Especially if keeping that, or other rich, grainy or high fat feed, I'd aim to feed his ration over at least 3-4 small meals daily. The more infrequent or larger meals are more problematic & less well digested.
I'm also not sure of the analysis, as the info page has 2 quite different ones on it, without explanation. Whether this feed or otherwise, I'd want to have a proper analysis, so I could work out what, if any, 'ration balancer' or nutritional supps were needed, as well balanced nutrition is vital for good health & imbalance could also be behind his 'failure to thrive'.