How to unblock a person?

Hi Team,
My request is slightly unusual - my son filed a report on Xbox user while playing Minecraft. This person is his cousin, they discussed the situation and would now like to continue playing together. Unfortunately I have no option to unban him (maybe he was blocked on the server). When he looks for my Gamertag it does not come up. When I look for his Gametag it comes up, but nothing happens when I tried to add him as a friend,
Is there any way this could be reversed so my son could play again with him?
Thank you
Mat

:)