Hi Sarah1,
There is a legal principle called unjust enrichment which may be of use to you here. It is based on the premise that one party should not be unjustly benefitted at the other party's expense. This arises for example in situations where there has been a failure of consideration - i.e. a person has paid for something that has not come to fruition. I haven't dealt with this area of law for a long time, so another forum member may have a better idea about equitable or common law remedies available to you.
As with outstanding debts, i'd say the best place to start is write a formal letter to your son and his ex requesting repayment of the money. If they are not forthcoming, send another threatening legal action (if that's the route you wish to go). If still no payment,
seek a lawyer's advice.