You are going to have a difficult time with this I'm afraid. While a verbal agreement to lend money is enforceable, you won't be able to prove the verbal agreement existed unless the person you are suing confesses and admits there was a verbal agreement. Otherwise, you would need your son's testimony
to prove that the agreement existed in the first place.
Now, it is not entirely impossible to prove it. You can get your son to sign an affidavit stating the facts of the deal, i.e., how much the loan was for and when it had to be paid back. Then you would sue the person that made the verbal agreement with your son. Not the bondsman and not the attorney, they were just acting on behalf of the people who hired them.
That being said, the court may decide that the affidavit is not enough proof to enforce a verbal contract
in this case. In the end it will be up to the judge. However, in my estimation, your likelihood of success is very very low.
Please let me know if you need further information.