why do dogs have such bad breath?

Brushing his teeth would be a good start. If you are comfortable doing it & he will let you, there are also scalers you can buy to scrape the tarter off their teeth. I'd also start offering him treats that will freshen his breath.

get the teeth checked out by a vet, they should tell you if you need to brush them. giving lots of bones should mean it isn't necessary though. the usual cause of bad breath in dogs is dietary. check the ingredients list of your dogs food, and try and feed a dog food that has as little chemicals as possible. the more natural the diet, the less nasty the breath, less nasty the dogs other end smells, and the healthier their poo is.

Brushing his teeth would be a good start. This will help. Also, examine his teeth, if you have never brushed them, he could have tooth decay or an infection in his mouth, that will also cause HORRIBLE breath, and needs to be fixed. Dogs can have tooth decay and infections in their mouths while showing NO outward signs to you - no whining, or anything. So take a look at those teeth. Then you can try dog treats that are meant to help with bad breath. These will help keep his teeth clean AND kill the odors that are down in his stomach that can also cause bad breath.