You definitely need to disclose the accounts that you have with your son as beneficiary. If the other account is your son's account, and contains money that he put in that account (as opposed to money that you put in that account), you likely don't need to list it - but I'm unclear what you mean when you say "I'm second in his account", so my answer could change based on what you mean by that statement. You don't file the FL-142. You need to have the FL-150 and FL-142 served on your spouse.by mail by someone other than you over the age of 18. Thus, he will see all the information that you put in the FL-150 and FL142. There are penalties for inaccurate disclosure or failure to disclose.
Please note that this answer does not constitute legal advice, and should not be relied on, as each situation is fact specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship.
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