macOS Messages app not sending to certain contacts

This problem started the other day when I was using the Messages app in macOS. One of my frequent contacts who I message all day stopped appearing as his contact name, and instead appearing has his phone number, as if he wasn't in my contacts. The little option to share my contact with him even appeared in the iMessage window with our chat. As this was kind of abnormal, I signed out of my iCloud account within messages preferences, and signed back in. Now his contact was restored, however I can no longer iMessages to just this contact. I get the red exclamation point "Not Delivered" message. I still receive his incoming iMessages in the messages app, as well as in messages on my iPhone. I can send him messages on the iPhone, but cannot on the macOS app. I can successfully send iMessages within macOS messaged app to every other contact I have tried so far, just not this one.

So far I have tried deleting his contact in iCloud and recreating a fresh one, completing signing out and signing back into my iCloud account on the MacBook, resetting my NVRAM, signing out and back my iCloud in messages preferences several times, and nothing has worked. I have tried every step in the official support docs about messages app troubles. Searching the forums and reddit, I see that I am not the only person for whom this problem has occurred, but I have yet to come upon any kind of satisfactory answer. Any help would be greatly appreciated.