I'm assuming that this is something that's recent, and that your horse has not had a consistent problem with bad breath.

Bad breath is usually caused by dental problems. If your horse regulary has his teeth checked and you know that's not the problem, their are several other causes.

If your horse eats hay with foxtail or barley grass in it, the seeds can get stuck in his gums and cause sores that ulcerate and contain pus that smells very bad.

The bad smell may also be coming from your horses stomach. Horses with chronic stomach problems have bad mouth odor. The stomach problems can be caused by poor quality feed or infection. Horses eating their own manure have extremely bad breath; if this is the cause you will most likely observe this behavior.

Since this seems to have come on so suddenly, I'm guessing your horse has eaten something that caused a sore in his mouth, and that's what smells so bad.

You should have your horse checked by your vet. If he does have mouth sores, there may be other more serious causes, plus he will best be able to treat them.

This is a link to information on possible causes of bad breath in horses:
http://www.juliegoodnight.com/questions.php?id=60