What kind of food does Presley eat? Is the bad breath new, or has he always had it?
Teeth cleaning is better started early, anyway, but I wonder if something else is going on. A sudden foul odor on the breath can be a sign of some illness.
Does he go outside? Could he have gotten into something, eaten something that made his breath smell bad?
Is it posible you are noticing his breath just after he eats a smelly meal, maybe a few hours later he will have sweet kitten breath again.
If you are very concerned you can bring him in for a check up. At that time you can receive instruction on how to brush his teeth, too. do not use human toothpaste. They make special brushes and toothpaste for cats.
Or you can just take a bit of gauze, wrap it around your finger and wipe off his teeth with that every week.
But most 6 month old cats do not ahve bad breath unless somehting's wrong. maybe he's got a baby tooth rotting in there or something.
As you can tell I'm only throwing out ideas for you, I don't really have any definite answer.
But we like questions. Ask as many as you want. If you feel the need to contribute tell us stories about Presley and his antics.