If he cannot make the payment the bank's security recourse is the van which you say doesn't have much value. Let them take the van!
If they sue him, you can have him file a bankruptcy and discharge the whole thing, amoung other dischargeable debts he might have.
You may also want to close his checking account and if he needs a checking, open up one in another bank and make it joint either with his wife or with you.
Take care.