Now, if Congress will just tell him he has to stop. Or is it the courts? In which case, whom has to bring the case to the courts?

No one's in a position to make him stop by simple declaration. Congress has the War Powers Act, but it's almost certainly not going to be enforced by any court, nor would anyone have standing to sue on it. It's almost uniformly been treated as a non-justiciable political issue:

https://scholarship.law.duke.edu/cgi/viewcontent.c...

Where Congress can "tell him he has to stop" is by simply not giving him the money to keep going. Not by affirmatively trying to stop him, but by simply refusing what he needs in order to keep going. Fighting a war costs a ton of money. At some point, he'll need a supplemental appropriation to fund the war effort. There's a very good chance that Congress is going to put the screws to him before giving him the money - and perhaps insist on an actual authorization of the use of force, which would require his Administration to actually persuade a lot of skeptical GOP Congresscritters that there's a "win" in here somewhere if he's allowed to keep going.