How can I stop my ex from using my car without my permission?

If your ex-husband is in an accident, then you could potentially be liable because you know that he is using the car and have taken no steps to prevent an uninsured driver from using your vehicle.  You can try to argue out of it, but it be a hassle and expense on you should someone try to sue you.
As far as what to do about him using the car without your permission... you need to have a serious discussion with your son about letting anyone, including his dad, who is not listed on the policy use the vehicle.  Your son may think this is about your distrust of your ex-husband... but the advice and effect would be the same for any driver that is not on the policy--- your son is exposing you to a financial risk by letting someone borrow his car.  Even the best drivers have accidents.  You certainly wouldn't hate one of his friends for having an accident--- because they do happen--- but they won't take the financial hit... you and your insurance policy will.  If your son insists that it is 'his car and he can let anyone drive it that he wants to', then you and he need to make arrangements to transfer everything into his name.  If he is adult enough to make the decision, he is adult enough to be liable for the consequences of his decision. You can still make the payment for him...but the account will need to be in his name. (Which is a more than fair compromise if you still want to help)  If your son has been brainwashed by dad and refuses to either (1) stop people from using his vehicle and/or (2) refuses to get these accounts/titles in his name, then you need to take steps to cancel the insurance and transfer the title out of your name into his name.  The final option is to simply take the vehicle back as it is still in your name.  None of these options are fun.. but they all involve necessary discussions to protect your financial future.

1) Yes, if you own the car--and if it's registered in your name, you are legally the owner--then if a driver who is permitted to use it, which includes being permitted by someone else with permission (your child), is at fault in an accident, you, as the car owner, would be liable, or responsible, too.
2) Since you are the owner, it's your car. Tell your child that if he/she lets the ex use the car, you will take it away and sell it--and if the child disobeys and let's the ex use it, follow through and take the car.