After enactment of the 2005 Bankruptcy Amendments there is an eight year interval between Chapter 7 filings, not 6, so he cannot file until 8 years and one day from his last bky. If you want to keep the cars you are likely to have to file a reaffirmation agreement, so those debts won't be discharged in your bankruptcy. If he would have filed only for the car loans, it would be best not to.
Finally, your 401k, as long as it is in the 401k administrator's account, is exempt from attachment by creditors (except the IRS and under some States' laws, the State Revenue Dept.). Once you withdraw 401k funds, it becomes difficult or impossible to segregate those funds from other savings, so if you still owed the bank money after filing your own bankruptcy and getting a discharge, the bank might exercise setoff against a bank account in which you had deposited some 401k money.