Your dog may have a dental decay issue so check for that first. After that, we started using Zymox, a daily water bowl additive and that helped some. You can get sprays but we have not tried them. Our little girl has had the problem since day one. The boy never has had it, go figure.
Typically, if a dog has bad breath, it is because of a dental problem. Having a dental done will help you start from a clean slate, and then you can focus on good oral habits. Teeth brushing, CET rinse, raw meaty bones, etc., etc.
Cynder has bad breath and it isn't because of her teeth, but because of her underlying esophagus issue. So if the breath is absolutely disgusting, definitely look in to an underlying cause and don't just assume it is "dog breath".
Here's a suggestion that may or may not be way out in left field. Do you live in an area where there's a lot of Lyme Disease? Has your dog be tested for Lyme lately? Bad breath is actually one of the vague symptoms of Lyme. Lars had it two years ago had it and was pretty asymptomatic...but he had bad breath which wasn't typical for him.
We didn't think much of it other than dog breath. He popped positive on a lyme test that became routine with the heartworm test that our vets give. I was shocked. We started him on Doxy and his breath became fresher as treatment went on. Just throwing it out there...
Hadn't heard about the Lyme symptom - that's and interesting tidbit to keep. I would agree you should have the vet check for dental problems and you might try checking your dog food and adding a probiotic to it's diet.
Switching to a Raw diet would help for sure but also There is a liquid teeth cleaner you can get at Walmart that is supposed to help. You just put a few drops in their water. I forget the name of it but I know it was at Walmart.
There are various reasons why your dog has bad breath. It might be caused by plaques or tartar and gum or dental disease. For long-term treatment for his bad breath issue you have to brush his teeth daily using toothpaste formulated for dogs. If the smell is still there you can discuss with your vet oral health products that is suitable and safe for your pooch.