We have all faced this problem.Tell him he needs to train more with a metronome or with you guys on timing in measures....and he needs to throw the time needed into his rolls.Practice makes perfect practice.
If you tell him this and he still...after a while doesnot improve...fire him.

I think it all depends on what you're playing and who you're playing it to.

If the problem is so bad that the bandmates can't keep up with the ever-changing bpm, then get rid of the drummer. It's a hard conversation to have, but that's the role of a band leader - to lead.

If it's only a slight annoyance and you want it to improve, then have the conversation, but don't just say "dude, get better" and leave it at that. Say "dude, we gotta be tight, and a big part of that revolves around you, and so we as a band want to work together with you to figure this out".

Both conversations require a bit of leadership.

(been a drummer for 30 years)
He needs to practice with a metronome if the rest of you rely on it. If not you all need to practice together. A Pro drummer can do either one. If you are paying this guy then what for? You can pay a real drummer that has worked hard to be able to do both.

Introduce him to the click. He will either learn to live with it or he'll quit.

Personally, I feel like the metronome sucks the life out of music. But your drummer must be able to keep a constant and steady meter also.

Oh and I almost forgot. Make sure he can hear what he needs to hear. a lot of the time bands think the new drummer should just know whats going on. He/she cannot perform well with a poor monitoring system.

practice practice practice!

Fire him. Hire me. The function of any rhythm section player is establishing a groove. If the house drummer can't play to a click, you should tell him find another home....