Could this have anything to do with his diet, the things that he likes to eat?
There is a condition called "Halitosis", otherwise known as a chronic bad breath. In other words, some people have a medical issue that results in bad breath. Or, he might just be eating smelly things, I'm not sure.
Look... he might be a nice guy, but this might be a deal-breaker. One of two possibilities here... either he has a medical condition beyond his control, OR, he needs to be willing to try a lot harder to have acceptable breath. Mouthwash shouldn't be a problem for anyone who is not a pussy. YOU are worth some mouthwash, or at least I should hope so...
If he is not making a serious effort, then this problem will never go away, and you will be unhappy with him. You have to think about your own happiness... and frankly, guys who don't make a proper effort with stuff like hygiene and breath are signing themselves up for failure in the romance game, and have no-one to blame but themselves.
So, can this work? Is the situation going to change? It doesn't sound like it can work... and changing that situation is his responsibility. I'm just saying... you shouldn't put up with this forever.