Well, he may have a cognitive or clinical hearing issue. If you really think that's it, then he needs to be checked out by a Dr.
If not, then it's a different problem.
Do you talk to him so much about so many things that he tunes you out? So, even if it's a big thing he isn't really paying attention?
I don't really know you well enough, but I would guess based on stereotypes that you probably talk to him like you to talk to other women. That is the single biggest mistake we, as women, make. It's also really really common. He's not your girlfriend or your gossip girl at the office. But if you yammer at him he will tune you out. Because he can't fix it, and his brain doesn't process information the same way that ours does so he can't listen to it. But then he gets so good at tuning you out that when you say something that actually IS important he isn't paying attention.
Talk to him less. Use fewer words. Get him to be curious and ask you questions. Answer only his question without the 5 minutes of details that we, as women, like to include when we talk to other women.
If you need to tell someone all the details about your boss's see-through blouse and what a tramp she is..... call your g'friend. If you want to figure out how to cook a standing rib roast and 4 sides in the same oven.... ask him to save your life and grill the roast. THAT he'll listen to. You've asked him to make fire and feed others.