I imagine if he’s serious he’ll contract his own solicitor, who will advise him as what is and isn’t advisable to request so as to best make it watertight. All the pitfalls posters are pointing out here, will also be pointed out by a halfway decent solicitor.
Marriage is a contract, and it’s usually women who are advised to marry in order to protect their interests. Prenups are just another, oftentimes sensible, means of protecting interests. This is generally why it’s better to work out these details before making yourself financially vulnerable by giving up work to raise children.
Whether he’s right or wrong in wanting a prenup is irrelevant at this point. He holds all the cards here. You may have enabled him
to continue his career, but presumably you also actively chose to have children and give up work to care for them? Oftentimes enabling the partners career isn’t a motivating factor for giving up work, that’s just a bonus to the main reason, which is that the woman (and it is usually the woman) wants to be a sahm. As she’s enabled him, he’s enabled her. OP is also responsible here for the decisions she’s made that have landed her in the position she’s in now.
The options now are to remain unmarried, and unprotected, or to instruct a solicitor to negotiate (this will probably be insisted on tbh, so as to ward off a claim that you were ill advised) for your own interests and see how that develops. Unfortunately you are in a much weaker position in regards to this, and that probably isn’t going to change at this point. What you can do, for yourself, is seriously look at what you can do to improve your own financial outlook so you can work at being less vulnerable, whether married or unmarried.