Grass and hay alone won't give him all the nutrients he needs, and that could account for the cracks in his hooves as he won't be getting the recommended amount of vits/minerals. In the wild horses graze on a wide range of plants to keep themselves healthy, unfortunately our grass kept horses tend not to have the same variety!
I'd recommend a low calorie feed balancer like Baileys Lo Cal, or Top Spec Lite or Anti Lam. They can all be fed on their own without anything to bulk it up, so it would be a small feed with not too many extra calories, and feeding the recommended amount would mean he was getting everything he needs. You won't see an improvement overnight as it takes a while for the hoof to grow down, it can be a few months before you see a difference but it'll come!
If he is podgy already at this time of year I would be thinking about restricting his grazing, either by keeping him off the grass for part of the day or by using a grazing muzzle.
I don't put anything on my horses hooves, I just make sure he has a really good diet and I don't need to. He is barefoot too and has great feet, and he's a really good doer so I understand your dilemma!