[Question] How can we politely address our guitarists timing issues?

I am in a three-piece band and our singer/guitarist is having some timing issues. He is used to being a solo act and is struggling to lock in with us on his rhythm parts.

He is much older and more “seasoned” then the rest of the band so it can be hard to critique his playing from our stance. but we came from an academic musical upbringing and have a greater understanding of listening to the ensemble and locking in. He’s a bit old school and stubborn about this topic. He insists that practicing with a metronome is counter-productive and sees it as a crutch. He is implying that if the bass and drums are locked in it should give him freedom to drag or rush the beat for “style and expression.” This may be true for leads/ solos but it’s really affecting our groove when he plays rhythm.

Has anyone been in a similar situation or had any luck communicating similar issues without causing hard feelings? Any help would be appreciated. He’s a very talented musician but this one specific problem is holding us back from our potential.