FaceTime icon missing with the only one specific contact

Hey

There is no FT icon with one of my contacts. We used to talk every single day till last week when FT icon disappeared. I can call anyone of my contacts so my FT works fine, he can call his friends as well. He doesn't have FT icon with my contact too. We don't know what could have happened, we tried to sign out and sign back in, turn off and turn on FaceTime, we deleted our numbers and added them again. I think there is something wrong with his phone because I asked one of my friend to add him and she can't FT him too because there is still no FT icon.

Any ideas? What can we do? It's very important to me because we can talk only by using FaceTime...

HELP!

iPhone 6+/iPhone 6s