I'm not surprised you are upset by this, I would be too. Prenups are not necessarily a bad thing but it seems you have,without any legal protection, trusted him for 13 years and now you stand to gain those legal protections he wants to make sure he's not vulnerable. It would perhaps have been less hurtful if this had come up in conversation when you decided to get married but 8 weeks before the wedding feels particularly calculated.
I wonder if he has discussed financial arrangements for you and the children before this? If his intentions are good then I would expect he has a will making provision for you all in the event of his death, if that is not the case then I would conclude he is motivated purely by self interest and I would be very careful before I signed anything.
I would usually say that honesty is the best policy but I think in this situation you could look at the prenuptial agreement and get legal advice without telling him you have done so. If he wishes to proceed he will also have to disclose all of his assets in order for the agreement to be considered in the event of a divorce, this could be useful information in the event that this becomes an insurmountable problem and the relationship breaks down before the wedding. It will give you the information you need to claim child maintenance at a level appropriate to his income.
I think it would be wise to keep a record of the dates of conversations with him about it as I would consider a proposal of this nature being put forward just 8 weeks before your wedding coercive, particularly after 13 years and 3 children.
If you decide to sign a prenuptial I would make sure that he maintains your NI contributions if you are not planning to return to full time work in the near future as this will affect your state pension.
I hope you manage to sort this out before the wedding and enjoy the day. Good luck.