Why is one of my contacts recently not receiving sent texts from me, but still receiving calls?

[AC Question]

My issue is that one and only one of my contacts is not receiving texts from my GS3. Phone calls from my number still go through to this contact, and I am receiving texts and calls from him. There is no indication on my end that the texts are not being sent, no continuous"loading" symbol and no"message failed" notice, he just doesn't receive them. There are no problems receiving texts from his other contacts, only from me. A couple months ago, I had same issue of not receiving texts from him, but it was only a couple texts here and there, and apparently resolved itself after less than a week. The current problem has been going on for about 3 weeks and involves ALL texts I send to him. We are long-time contacts, with no recent number, device, or provider changes for either of us. Does it seem the issue is on his end or my end, and what to do? Thanks