Thank you for giving me the opportunity to assist you. I encourage you to ask me for clarification, if you are not clear with my Answer.
Go back to the Court where your divorce was granted and give the Clerk your docket number and ask to review financial statements filed with the Court. The financial statements are impounded, that is, they are removed from public file for privacy reasons. His social security number should be there. If you did not file financial statements, then try one of the People Search sites. They may be able to provide someone's social security number at nominal price:
Child Support Enforcement Services ("CSE") should be able to get his social security number. That is the only way that they are able to intercept his tax refund. Anyway, you should try the People Search sites that I provided you earlier if CSE does not have the number