I have a joint with my 16 year son. His account was frozen due to my garnishment because I am a co signer on his account. Can the debt collector seize his money?

Yep. Once they obtain a judgment, they can attach any asset that you are listed as a co-owner. If you can prove that all the money in that account came from your son's income, you might be able to get the attachment vacated. Otherwise its will be turned over to them.

You have to be a listed owner of the account, so you have a joint account with your son.

Your son can file in court and prove that all or part of the money is his, that he earned from his own jobs and working