Without knowing what and how much grain you fed earlier, I'd still be reluctant to put him back on grain. Once they founder, they are more prone to do it again. His weight does look good in the last 2 pictures so he's getting the right amount of calories. Other options to get the nutrients you are looking for by feeding grain would be a loose vit/mineral blend (no sugars and virtually no calories) or a ration balancer while upping his hay. Even if you get up to hard training and he needs more calories, feed him oil, BP, alfalfa... anything with more calories without the sugars of grain.