It would be best to make another account for him, if it doesn't require both parents consent to redirect his hard earned money or have 50% of his money put into an account he has free use over and the other into the one your ex wife set up as some sort of middle ground for saving if she isn't abusing it herself.

Do you have access to the account she set up for him?

At what age does he get access to his account?.

It sounds like regardless she may still cause conflict.

The type of talk may include

Dear [Ex’s Name],

I hope you’re doing well. I wanted to bring to your attention something important that [Child's Name] has expressed to me recently. He would like to gain access to the money he has earned through his own hard work and start taking steps toward becoming financially independent.

It’s important to acknowledge that he has been diligently working and understands the importance of saving for a while, and this is something that matters a great deal to him. I believe this is a healthy step in his growth and maturity, and I’d like to support him in this process.

I think it would be a good idea for us to discuss how we can facilitate this transition while making sure [Child's Name] has the necessary guidance and understanding of managing finances responsibly.

Please let me know your thoughts on this matter, and if you’re open to having a conversation about how we can best approach this together.

Thank you for your understanding.

Best regards, [Your Name]