Half an inch isn't anything to freak out about, you just need it to be taken care of so it won't get any worse and eventually cause a lame horse.
I would recommend not riding him until the farrier trims and files his hooves. As for the farrier; he will probably trim the hoof down like it's a normal trimming, then file the hoof up some to help heal the crack. Since the crack is only about half an inch, one trimming should help wonders. And I'm sure you could probably ride him after the farrier fixes his hoof.
To prevent future cracks, you can keep his hooves conditioned; a wonderful way to keep them from cracking. It's kind of like dry skin. Put lotion on and you will fell better. You could probably buy some hoof conditioner at a local equine store. Massage the hoof dressing on the outside of the hoof, then thoroughly pick out the hoof and rinse it with water on the inside. Let it dry, and massage the hoof dressing to the frog, hoof wall, heal, and coronet. This will prevent cracking and it will stimulate future healthy hoof growth. However, do not apply hoof dressing TOO often because then the hoof could lose the ability to absorb moisture itself.
Another way to prevent cracking is mud. Surprisingly, when horses stand in wet mud their hooves are soaking up the water. Maybe, by the water trough out in the pasture, you could make a small mud puddle. So when your horse goes to drink water, it will stand in the wet mud puddle and his hooves will absorb the moisture in the mud.
I do these methods to my horse very often in the summer when the southern heat gets to her hooves. It helps a lot.
And remember, it is always, ALWAYS, very crucial to keep the hooves trimmed and at a good length. A way to tell if a horse's hooves aren't to long is if the heals of the hoof are off the ground.
http://www.danzaanddanza.com/farms/customs/grey/raja/hooves.jpg
See how that horse's hooves are more upward on the toes? And not back on the heal? Those are nicely trimmed, and very well taken care of hooves.
Here's a picture of a horse's hooves that are too long.
http://media.photobucket.com/image/hooves%20too%20long/NecromancerAda/031.jpg
Very poorly taken care of hooves. If the heal is almost to the ground like this horse, then they are too long. Sometimes, it will take many clippings and farrier visits to get hooves like these fixed. Hooves that get longer then this could eventually cause founder.
So my advice to you would be to not ride him until the farrier trims his hooves. Keep his heals off the ground and his hooves trimmed every 6-8 weeks. And keep them moist to prevent future cracking. I hope everything works out. :)