Writing a recommendation letter for a senior colleague
The fact you are more junior does not mean you can't write a helpful letter.

Write a letter that describes what you observed, e.g., in his lectures and in his interactions with students and colleagues, and in his research.  Try to be specific.  Instead of simply saying he was a great lecturer, describe the techniques you observed, e.g., that he mixed PowerPoint and whiteboard effectively, answered students' questions really well, how he's mentored and taught you, etc.

After you've outlined the facts that might support it, offer your opinion, that this is an individual who would be perfect for the position.

I'm a lecturer in CS at University of Michigan.  I've written lots of LORs for both students and colleagues.  They're very formulaic.  You can do this.