In order for the bank to go after any of your sons personal assets, other than the property subject to its lien/deed of trust/mortgage, the foreclosure will have to result in a deficiency.
At that point, the bank would have to seek a deficiency judgment from him. If granted, the bank would have the right to liquidate his assets and apply them to the debt. Usually, the bank will try to garnish wages first. If this does not work out, it will look for personal property and possibly other savings accounts.
However, a safe deposit box is not an account. It's generally where you keep important documents and possibly cash money. If the account is held jointly, your son can claim that the money in the account belongs to you - if it is cash, there is no way the bank can prove the money is his - unless he confesses the same.
If there were some other form of money in this account, the bank would only be entitled to whatever part belongs to your son. For that, the bank would have to depend on the statements of you and your son because the assets are comingled.
Therefore, it would be next to impossible for the bank to get the money, and it is likely they won't try because it knows of the headaches created by joint accounts partly belonging to the debtor.
All that said, it is possible for the account to be subject to the creditor/bank, but it is highly, highly unlikely.
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