From a man's perspective there are two issues here:

1) He is not properly acknowledging your concerns 2) He may not know how to fix the problem

The answer to the second problem may actually loop into the first problem, and here's why: he may be taking some steps that are normal i.e. brushing teeth and normal dental care regularly, but it's not helping so he thinks it's impossible or a problem you're making up.

The problem may not be dental but some issue in the throat -- which I have experienced. One big contributor to halitosis is Tonsilloliths or tonsil stones. This would explain why "There is still a smell after brushing but it's a lot less" as you mentioned elsewhere. Aside from tonsils, there can be times where pockets are formed in the throat where debris will collect and bacteria feeds on it causing bad breath. Unfortunately, halitosis is always contributed to dental hygiene when that is not always the case. Therefore, brushing or mouthwash before sex is just a bandaid for a different problem all together.

Therefore, it may be less of an issue of him not acknowledging your grievances and more an issue of frustration. He may actually put what we would deem "normal" effort into brushing and going to the dentist, but no amount of normal dental hygiene will solve a problem originating in the throat. And if he is doing these things for you and and all he keeps hearing is how it's not enough, he may be getting frustrated and thinking that it obviously must be something else, which is how it's affecting problem number 1.

I would highly highly recommend he go see an ear, nose and throat doctor concerning this.