Get rid off Montesquieu. Accident, disease, whatever. It's ironic that a champion of the separation of powers managed to get the Estates General into such a powerful position that the King couldn't govern without their consent. Additionally, the reform of 1768, which merged the First and Second Estates must be stopped at all costs. Keep the structure of the thing as complicated and bureaucratic as possible. Prevents the assembly from effectively putting demands on the monarch. If you want to do it really late, then your only chance is stopping the reforms of Council-President Célestin Lanova in the 1840s. Once Senators could be elected to sit with the noblemen and bishops in the High Estate, there was no going back. Have King Philip VII oppose the reforms and rally the old order against Lanova. Or even better, make sure Lanova was never born and the Parti conservateur-radicale never find a leader as eloquent and commanding as him.
As it was, by the early 18th century, the French Constitution established the King to rule together with his Council, and the Estates-General passed the law. The King would traditionally appoint a prominent figure from whoever held the majority in the Third Estates as Council-President. Due in part to the Council-President's command of the Estates, as well as Louis XV and Louis XVI's preference for parties and other frivolities over the affairs of government allowed the CP to establish himself more and more as a figure of authority. In the 1790s, the English satirist Frederick North marveled over that France seemed to have "two executives" However, the gradual establishment of a partisanship in the early 19th century was growing more and more problematic for the established order, and Lanova encountered severe difficulties in pushing through for representation of the colonies and in ending the trading embargo on the Third British Republic. Although he had the support of the Third Estate and much of the bourgeois aristocracy and landowners (who benefited greatly by selling corn to Britain), many in the old order hated the idea of British manufactured goods flooding the poorly industrialized France. This would only bring about more power to the wealthy layers of the commoners, and thus undermine their own power. Despite re-election after re-election, the King had no choice but to re-appoint Lanova. In 1846, things drew to a close and Lanova introduced the Senatorial Act, which effectively halved the representation in the High Estate as far as the nobility and the clergy were concerned and introduced elected Senators. After the bill first passed the Third Estate, the High Estate vetoed it, despite Célestin Lanova's ally Guilliaume de La Fayette's very thorough attempt to bring it through. The King demanded a re-election over the bill, which the Parti conservateur-radicale won again. Finally, in 1847, the bill passed through the High Estate thanks to La Fayette and, with the Council-President having signed it, it was now up to the king. Philip VII, the old aristocrat, now refused to do so, creating a constitutional crisis. In early January 1848, the Bastille was stormed, and massive protests were held all over France. In March, the largest of them all were held outside of Versaille, and it continued for over three weeks. On the 22nd day, the King invited Lanova to sort things out. When the Council-President was arriving, the masses started cheering and applauding, chanting "Célestin du peuple!" The King demanded over coffee that Célestin bring the country to rest, and expressed his intention to have the mobs taken away from the village of Versaille by the army if necessary. Lanova merely responded that only the King could bring the crisis to an end, and that by signing the bill. If he didn't, and tried to use the army, they would likely mutiny against their sovereign. Apocryphally, Lanova is said to have stated that unless the king signed the bill, he would surrender the State into anarchy. When the king responded by stating the words of his ancestor, Louis XIV that he was the State, Lanova calmly replied: "Non, Votre Majesté, vous êtes simplement leur servante." ("No, Your Majesty, you are merely her servant.")
Bullied, and without allies, the king signed the bill two days later, and soon enough the discontent ended.