Kinda skinny??? While they're not the best pics, am I the only one who doesn't think the horse is skinny, in any of the pics?? I know many people are accustomed to seeing roley poleys, but that horse looks a healthy weight. He does look like he's lacking muscle tho, esp on his topline & hips, which can make them appear 'thin'.
I definitely would advise you stop feed him buckets of grain, especially not corn. As someone pointed out, that could be part of the reason for him having probs keeping weight on. What was your reason for feeding him grain? Take a look into some equine nutritional info to learn about the various problems caused by grain(esp corn) & other sugary/starchy feeds. I would also be particularly hesitant about these ingredients given that his feet look problematic anyway. If for whatever reason you feel the need to feed him grain, oats are about the safest, most easily digestible, and they should be fed in very small amounts, at least 4 times daily, for them to have much benefit and as little detrimental effects as possible.
I'd be giving the horse ad-lib hay, turn him out full time with some mates, to ensure more exercise(& wellbeing generally), ensure he's getting well balanced nutrition, in the form of a *good quality* supp, and I'd also be feeding alfalfa hay or such, for the extra protein, for muscle building.