How do you ask a coworker to be a reference?
Try to use coworkers that aren't just your “work friends,” though. Ask people that you've worked on projects with to act as your reference. They will (hopefully) discuss the positive contributions you made to the project and mention that you're a fantastic team player.
How do I ask someone to be a reference on indeed?
How do you casually recommend someone for a job?
Write a letter for your friend to attach to his application and suggest he mention your name and recommendation in his cover letter. In a small company, talk to the boss personally to say you'd like to make a recommendation via a personal introduction. An informal coffee or lunch meeting can get the ball rolling.