Right so time to start seeing some lawyers. He's massively ahead of you and you need to play catch up fast. Do not go for one appointment see at least 2 preferably more lawyers. Do not worry about the cost. Every lawyer you see will be one that wont be able to represent him. It took me 4 lawyers to find the one.
Assume he is already with OW - if not today then very soon.
Start looking for and copying all the paperwork you can find.
Open a new bank account online away from your usual bank and put in as much money as you can. Move any child benefit payments etc to it. You can also transfer half the contents of joint accounts but that will give the game away so the timing needs to be carefully done.
Make sure you have a clean email address that he does not know about or have access to via other devices.
Assuming you've a smartphone download Evernote and an app like Scannable which will scan documents using your phone straight into Evernote. Evernote is accessible from everywhere and syncs automatically. (You might need to upgrade it but I think it's good value for money.)
Mortgages
Bank statements (back as far as the business sale if possible)
Passports
Investments
Car, bike, boat, racehorse etc big purchases
Pension statements
And anything else you can think of.
Any receipts for dodgy purchases - so not for you or kids
Then write a time line of what you know.
So married in x
House bought in x
DC1 in x
Sold business in x
This helps the lawyers and is useful for you in working it all out.
Leaving is going to be very simple for him because he already has a plan. His nest is probably already feathered. Good luck and do as much as you can as soon as you can. Save the timeline for when your brain is whirring at night, so you can keep adding things.