You need to either invite him at character creation, or invite him when you have more than one party member.
The way multiplayer works is the client is basically joining the host on their save - They don't bring their own character. If they join during character creation, they can make a character then, but joining during an ongoing save means they have to control one of your preexisting party members. If you only have one character at the time they try to join, your group is "full" then (since you're controlling the only available character).
