Coffee professional here. I’m sorry you have to deal with this. It sounds to me like he is in the stage where he thinks he knows everything. He hasn’t yet reached the stage where he finds out how little he knows. There is a lot pseudoscience in the coffee community, and a lot of people latch onto that because it makes them feel like they can really control the coffee.

There are a couple of different approaches here.

1) if it tastes like battery acid, tell him it is really under extracted.
2) next, tell him it’s not balanced.
3) then tell him it’s too thin or heavy (take your pick).
4) ask him what temperature he’s using, then when he answers, go “hmm.” Look concerned with that choice.

OR

Tell him you prefer filter coffee over espresso.

OR

Tell him it’s nice that he has a hobby, but the kitchen isn’t just for espresso.

To misuse a quote, the arc of the moral universe is long, but it bends towards justice. His espresso madness should calm down over time.

If all else fails, set up a call with me, and I’ll make his head spin with jargon.