iMessage isn't working for one particular person

I know there have been quite a lot of posts about this problem, but I couldn't find any that were exactly like mine. All the other ones have something about the text messages being green, but mine are still blue. My iMessages with said person have been completely fine for at least a year before I found out that my messages weren't being delivered, but the messages were still the same blue as when you send a message to another iPhone user. If I remember correctly, he can receive my messages, but if he sends one to me nothing shows up. Because iMessage doesn't work anymore, we've had to use other apps, like social media, to keep conversing. But I'm not gonna lie, it's really annoying, so if any of you know ANY possible solution, I would greatly appreciate it. If you need any questions answered then ask away.