you need to give him something to improve the horn quality, I had the same problem with not only cracks but brittle hooves. I've found that feeding a good hoof supplement plus micronised linseed (a mug a day) helped me.
What are you feeding? I also found that pasting on lotions and potions makes the problem much worse.
Just realized I assumed you meant toe cracks. Where on the hooves are they? Quarter cracks can be due to excessive wall length at quarters. Ime cracks often (just to annoy A Guilding lol) have a dietary component and that usually includes excess sugars which upset the hind gut and therefore lamina. A decent balancer will help too.
OP - as amandap says, not all cracks are created equal.
Posting some photos would be great.
Otherwise a bit more description about the nature of the cracks - superficial outer hoof wall only, more extensive involving inner hoof wall and/or white line? Concentrated around toe or quarters or both?