My vet also recommended farriers formula for Mitchell on Tuesday (He has a hoof abcess) she said he needs biotin to help strengthen his hooves which should help prevent abcesses in future.. So something with a good source of Biotin would probably be a good start for your horse by the sounds of it.
As for the cracks, this won't make them disappear magically, but "Worlds Best Hoof Oil" is really good for cracks, it does help them to grow out better, and it also fills the gap a bit.
Otherwise, if the cracks are really big or whatnot, you could clean the hoof out (possibly soak, to get the crap out of the cracks, bearing in mind this will make the hoof softer for bit though) and fill the crack with stockholm tar or something similar, to stop mud and nasties getting in there while it's healing.
-Just ideas from what I've had to do before :)
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms