Rocket League PC – How to Play Offline Multiplayer

This answer is for PS4, but I expect it works similar on PC.

While in the main menu, there is an indicator in the lower left corner telling you which local players have connected already. It also tells you "Press options to join". After every local player has joined this way (pressing the options/start button), you can simply start an exhibition, seasons or online match (except ranked) and your couch buddies will join automatically.

Tip: You can also disable bots by selecting "No bots" as difficulty in the match settings.

Invite him to your party, select Play Online, and select Private Match.