Connection Rejected. The chosen game is full

Hi All,

Some help would be appreciated if you can spare a minute...

Trying to play Multiplayer with my son - can't get connected.

I can see his game (and vice versa) in the LAN lobby, and I have tried IP direct also. In both instances I get the error
All I can do is click OK, and then get kicked back to the same screen.

I've been chugging away at this for hours, so perhaps I'm missing the obvious. How can I make a game accept more than one player?