Do we have grounds for a legal judgement against my ex-husband to pay my son's savings back?

My son was given an $11,000 savings account by his grandmother to use when he turned 18. She allowed my ex-husband and I to borrow it for a down payment for a home, with the ageement that it would be paid back with interest when the house sold or when he turned 18. When we divorced, it was in the court papers that he would be paid out of the sale of the house before we divided that balance. This did happen, but the money was place into a cd at the credit union that we had shared an account at prior to the divorce. My ex-husband had kept the account in his name and removed mine. The cd was put in my son's name as well as my husbands. It was supposed to be in my sons name only, but since he didn't have an account, it was put in my ex-husbands name as well. After repeatedly asking for the cd so that we could start an account in my son's name, my ex-husband finally admitted to spending all of the money. What legal recourse do we have? Where should we start?