The only reason I have the money that I do is because after our previous house sold, my therapist suggested I transfer some of the money (which was deposited to our joint account) into my own account. Max quickly put the rest into his. He has an online account with no paper trail. I don't know yet how I'm going to get it accessed.
If he got even a dollar's interest, he should have a tax form from the bank. Your lawyer will have to demand his entire return, forms and all, and it would be a good idea if you knew which bank's paperwork to look for; check his browser history. He should be warned that to hold back funds will be, at best, perjury, and that he will be prosecuted if discovery turns up undisclosed accounts. My guess is that more money is moving around than you're aware of.
A forensic tax attorney would be helpful. It's no longer possible to open a bank account without disclosing an awful lot of information.
The thing about a bully -- and he sounds like one -- is that if they realize they're overmatched, they'll just run. Get very tough people on your side and prepare to be very tough. Tough custody lawyer, tough financial person. Don't back down, and don't be sweet-talked into over-generous concessions. Stand firm and encourage the people you're paying to do the fighting. Also, talk to your family and see how far they can back you up as far as paying lawyers goes.