Fairly common? Sure. Not a concern? For me, it would be.
The lean will cost him at release point - it will pull his release point back. House claims every inch of inappropriate head movement costs 2 inches at release. Some folks don't value getting the release point closer to home plate but it gives the batter less reaction time, creates later movement, and creates better movement due to getting to a stronger position to spin the ball.
Also, the lean can represent a misdirection of energy with his center of mass moving one direction while his arm tries to throw the ball in a different direction. This can affect control and velocity and can possibly become a health issue.
The first step to correct it would be to look at stride direction and starting position on the rubber. He could be setting himself up to lean right from the get-go. If there is an issue there, then the fix could be as easy as changing starting position on the rubber. It's easier to change position than movement.