Topic: Lego Super Hero's Co-op

I'm trying to play 2 player with my son, he only has one joy con, in the game it says to press + to add second player but this just pauses the game. Please can someone advise how/if it's possible to add a second player this way?

I don't actually have the game, but based on the Wii U game, you have to press +/- on the controller you wish to add, not on the one you are already using.

I'm not sure if I explained properly. We only have 1 joy con which is split into 2 so the - button does nothing and the + button on the just pauses the game. The game automatically launches as 1 player, presuming you are using both sides of the joycon so in effect there is no side I'm not using. Please help 😭