There’s nothing you can do but tell him it bothers you and it’s a problem he needs to address, or find another drummer.
Ive had to have that conversation a hundred times as a band mate and as a record producer, it’s never easy, feelings will get hurt, accusations will be made, excuses will fly, and unless you’re really committed to the guy... it’s similar to getting someone admit they have an addiction, or if you had a tubby girlfriend that would be perfect if she lost a hundred pounds.
Now, you can put in the effort, practice to a click, record your rehearsals and shows and have listening sessions where you point out fluctuations, buy this box called a “Russian Dragon”, and work with them. It is possible you’ll help them, and that they’ll get better...
but then just like the addict that got clean and the tubby chick that’s now “hot”, they’ll leave you in the end. Because now they’ve got their sh!t together and they’ll be attractive to other bands, and they’ll most likely carry the resentment of you with them until their last days, and they’ll have something to prove to you and themselves, so they’ll get a hotter boyfriend or band.
I’m telling you... without a shred of exaggeration or hyperbole, I’ve seen and been through this tens of times, and always to the same end. Find another drummer, or adjust your expectations.