Does a certificate of release mean they won't arrest him for DUI or is that still possible?

My son was in a single car accident and had to be extricated from the car. He was asked by the officer if he had been drinking and he admitted that he had. He was given a breathalyzer and transported to the hospital but before he left the officer took his license and told him he would need to wait 30 days then pay $125 to get it back. He also gave him a "Certificate of Release" that said he was detained but not arrested. He was released from the hospital and hasn't been contacted since. Maybe he wasn't legally drunk? He also didn't receive a citation. How do we find out?