Ideally you should have been setting aside money from your previous paychecks in anticipation of the day that the employer would finally do the proper withholding because you always owed that money and that should have been built into your planning. One can only assume that if he wasn't taking withholding out of your checks properly, he also wasn't doing the employer share properly as well so it is in the best interest of the business to correct all the errors ASAP. If it isn't taken out in a timely manner, you could face issues of underwithholding when you file your 2022 taxes and that could result in a penalty (a small one, certainly, but a penalty nonetheless).

Regardless, here we are today and you're faced with potentially being short. There isn't much you can do as your employer is likely within legal limits to take it all at once as a correction. You can always ask that it be spread out, but that is risky for the company as these errors need to be corrected as soon as possible so he can always say no.