Those types of cracks are nutritional/diet related, so as others have suggested, a*good* supplement is the way to go. But biotin & the ingredients in many 'hoof supps' are only but a few of the necessary nutrients, and it is also important for them to be well balanced with eachother. So unless you know he's short in only a few ingredients, I'd be going for a good complete supp. Hooves & hair are the extremities that are the first to show bodily deficiencies. It will take a while for the new, well fed growth to come through. Depending on his management, feeding & especially the amount of exercise he gets(the more miles the quicker the growth), it will generally take anywhere from around 7 months to a year to grow a whole new hoof capsule.
Oh, why I say 'diet/nutrition' is because he can have diet issues aside from nutrition - it's obvious he's had a couple of bouts of lami, from the bulgy growth rings, so perhaps he's been getting too much rich, sugary feed, such as sweet feed, cereal grains, rich grass...
Regarding topicals, I also recommend just water, or water with a little apple cider vinegar in it. I'd stay away from oil based especially & turps! If it's really dry where you are, soaking them or allowing him to stand in a mud patch every other day may help. But dry hooves are a natural state, and so long as they're fed right and balanced right, they won't crack. Hard, dry, calloused frogs are the way they're meant to be & will be capable of a lot more for him than the soft, weak frogs of most pasture dwellers.
Cracks also happen due to imbalance & allowing the hoof walls to become too long. This is not the case for your boy's type of cracks, but there are a couple of issues that could be rectified before they cause the cracking to get worse. That is that that side of his toe where the cracks are worse appears to be slightly longer/pointier than the rest. Needs to be 'rolled' back a bit more. And there are minor quarter flares, indicating that the quarters have been left a little long & are just starting to 'give'. Can't comment any further on hoof form with only that pic, but if you want to send any more for comments, feel free.