General Discussion

There are 9 people charged with deciding what the constitution says. Currently, 5 of the 9 who get to say what the constitution means agree that - with few exceptions - being born in the United States confers citizenship. Four of the 9 disagree with that interpretation.

Saying he disagrees with the majority doesn't mean he believes that Congress can override the constitution (your contention). It means that he disagrees with the current interpretation of the majority about what the constitution says on birthright citizenship. He does not believe that the constitution confers citizenship on people Trump's executive order tried to exclude. He believes that citizenship for that group is conferred by statute, not the constitution. If citizenship for that group of individuals is conferred by statute (but not by the constitution) - then Congress can change the statute. Because Trump tried to change it by executive order, the order is invalid.

As long as the majority holds, if Congress passes a new statute, the court would rule that the statute limiting citizenship is unconstitutional. Kavanaugh would almost certainly dissent, not because he believes that Congress can override the constitution - but because he simply doesn't believe the 14th amendment grants citizenship to the group Trump wants to exclude. If he is correct (and with a shift of one justice he would be), all that is needed to change it is a statutory change.