Hi, thanks for writing.
It is common for criminal defense retainers to have payments made throughout the entire period of prosecution. In which case, he may have "re-signed" or fully agreed to pay the retainer at one point when he did turn 18. This could be done outright by saying it or signing it, but it could also have been done impliedly by continuing to use his services once he turned 18. Your son, on his 18th birthday, could have fired the attorney. He did not. So it is plausible that the attorney found your son impliedly agreed to continue using the attorney's services once he turned 18. In which case, he would be liable for the amount owed.
That being said, you, as co-signer, would be jointly liable for the amount owed. I'm surprised the attorney hasn't gone after you for the remainder of the money owed. He likely will one day.
In general, a person under the age of 18 cannot be bound to an agreement/retainer/contract.
Evan H. Nass, Esq.
Answered on Mar 15th, 2013 at 3:07 PM