You better take your cat to the vet. I didn't have this problem but my vet
was promoting dental health so for $90.00 I had Isis' teeth cleaned. The vet
didn't find any cavities. You should have your cat checked for cavities and
gum disease. An extraction might be required if a tooth is decayed.

Mike in Illinois

I would first check the food and/or treats I'm feeding him, but if that
isn't the source of the smell then it might be a good idea to see the vet.
While kittens dont usually have a lot of tooth or gum problems it is always
possible.  There are also other health problems that can cause foul breath.
Also, double check to make sure he isn't eating anything he isn't supposed
to.

My cats are reporters, tell all your feline friends about
http://www.catnabbit.com

Just like in humans, halitosis can be idiopathic, and if it is, no measure
of "breath freshener", or tooth cleaning will get rid of it. Try a different
cat food???

Good advice given here.  I believe some human mouthwash has alcohol and
since we can't tell the kitten how to gargle I'd stay away from any
mouthwash with alcohol.

My late kitty had bad breath which turned out to be a complication of
renal disease!  Go to the vet!

Well, the vet gave Ornery a clean bill of health..although he is 9 months
old and weighs 10.25 lbs..so will need to watch his eating habits or he will
be one fat kitty!
She gave him an anal gland exam to be sure he wasnt having any problems
there causing the foul breath. She concludes that he must be eating
something nasty or I just get a whiff of his breath after he has cleaned
himself post litter box.