I’m sorry this is happening to you - it’s honestly a betrayal at least as bad as infidelity.
Some steps, because when someone has lost a lot of money, they very seldom want to admit exactly how much straight away.
First, check all bank accounts, statements, credit cards for the entire time period this has been going on and identify the transactions or transfers. That’s his accounts, joint accounts and your accounts too. Make a note of how much, anything identifiable and any patterns. This will help you to work out the scale of the loss and also flag anything that happens going forward.
Second, credit checks for both of you so you can work out if he has put debt in your names.
Third, check for charges against your home and whether that is now at risk directly. If you’re also on the mortgage and he has done so without your knowledge then that’s fraud.
Fourth, the other parties involved. Has he borrowed their money and invested it on their behalf or his own? Or had he simply convinced them to invest in the same thing as him and they did it themselves? That part is quite key when it comes to whether they can sue for the money.
He will likely drip feed or minimise it and it may take some time for you to find out exactly what has gone on. That the police are already involved suggests it’s pretty serious.
Next steps - ringfence your own accounts, hide or cut up credit cards, get him off any joint account. Set up an alert on your property (land registry) so you will know if anyone tries to place a charge against it. Keep on with the regular credit checks. If there is also debt involved, be aware bailiffs may turn up depending on what stage it’s at.
Unfortunately scam victims, much like gamblers, can be in denial and keep throwing money after it convinced it will all come right etc.
This scale of financial stupidity would be a deal breaker for me - placing your and your children’s home and financial security in jeopardy would be the end. I’d be doing the above checks and then looking to divorce.