I see two relatively non-confrontational ways to deal with this situation.
Repeat the message of a slang phrase with a more standard/classier formulation.
Pretend you don't understand the slang phrase and ask him to clarify it. Keep doing this, even for slang phrases he already explained. Maybe he gets annoyed and starts using standard English (with you).
There is no plus side to getting involved. Probably everyone else notices as well but is staying out of it.
This sort of behaviour can be career limiting in many places. You could give him a heads up if you felt the need, but that has potential issues, so you're best just to try and ignore it.
Acting professional is true for almost everyone. There is an ...
Tell him to knock it off because it makes him sound stupid.
Honesty still is the best policy.
Like in the automatic suppression layout editor on this word processor stinks, because it glues sentences together that don't belong together, thereby damaging the message, which is kind of,.. stupid.
If I was him, I would want to know.
Maybe don't tell him in front ...
I think you answered your own question:
He can act professional when he wants to
Operative word there is act. He can act professional when he wants to. It's an act. That's not how he is naturally or normally. And you're asking him to, in addition to doing his normal duties, to act in a way that he normally doesn't, and pay attention to that.
People are ...