my son had permission to borrow our van but then he left and we had no contact with him for several years. Now the van has been impounded several stat

If the van was titled in your name and you knew your son had it and did nothing to get it back, YES, you are most likely legally liable for what he does with the van. You should have addressed this 4 years ago, and likely reported it stolen, hired a repo agent to recover it, or at least cancelled the tags on the vehicle so it could not have been driven without law enforcement stopping him and seizing the vehicle. You will need to contact the tow yard to address payment of any balance and recvoery of the van or surrendering it to them to pay for the balance. Otherwise, they might sell it and if there is more you owe the could try to have your DL suspended here in Florida for non-payment by statute.