You answered the question yourself - the court will have to resolve this. First, since your son is no longer a minor, he can certainly try to obtain the account for himself. The alternative is that you take your ex's name off the account via court order.
This answer is not to be considered a response to a specific legal issue in a specific jurisdiction - it is to be considered only a general response to a hypothetical scenario posed by the questioner. For specific legal advice, please consult with a licensed attorney in your jurisdiction.
Sign up to receive a 10-part series of useful information and legal advice about the divorce process.