I've done Team Create with my brother a million times but for some reason I can't find the text box to add him. https://gyazo.com/135e2ce275ec83a1b25d849de4dd3cbb I don't know if I'm being stupid or something but I seriously can't find it.
You need to go into Home -> Game Settings -> Permissions then enter his username into the box. This is the way mine is too.
This could either be this is a group game, or you have to do it through Home -> Game Settings -> Permissions. If you have the game under a group he can just click edit on the game as usual.