This could be a couple of things. The most likey cause is a bacterial or fungal skin infection. This can be a problem even if you see no redness or itching. You should see the vet to rule this out as a cause.

Another problem might be the dog's anal glands. Dogs have two glands at 10 and 2 like a clock beneath the tail. These glands secrete a smelly substance every time your dog poops. Sometimes these glands leak this foul substance and the smell is pretty bad once it's on the dog's skin. If you suspect this, you can use a baby wipe or vinegar wash to clean the skin on his bottom. Your vet can express the glands for you and hopefully take care of the smell.

If the odor is on the upper part of his body, I would feel led to think that he might be licking his anal glands and you're smelling the foul odor on his mouth.

Also, I would be concerned about the dog having yeast in the ears. This can cause an old dirty sock or locker room smell or it might even be a pungent, sweet smell.

If you have not tried it yet, you can wash the dog in a vinegar solution to combat the smell. Douches like women use work well for this because they have an applicator and you can spray the liquid onto the coat and reach hard to get to spots.

Let me know if you still have questions.