Is this legal and why would he call him on the phone and ask him to return to the Taco Bell and not issue a citation for the accident?

My son was in a minor fender bender at a Taco Bell in the drive thru. He and the other party exchanged information and because there was very minor damage decided to handle this privately. After all information was exchanged, and both parties agreed they could part ways, he left the Taco Bell,. About 10 minutes later, and after he was home, the party that they were involved with called him and said there is a cop here and they want you to come back. He did and didn't think anything of it. The officer asked him what happened. he never issued a citation for the accident, he asked him if he had been drinking? He told him no not for the past several hours. He did agave a couple of drinks at dinner earlier that evening around 8:00. This is now 12:30 am. He gave him a field sobriety test, which he said he did fine on and then asked him to do a breath test. My son asked to do a test at the hospital, at that time the officer became annoyed and told him he was under arrest and hand cuffed him and put him in the back of the police car to take away. When my son's girlfriend ( they are college students, living in Athens, Georgia) asked what he did wrong or what he was under arrest for the officer would not tell them and just said that he was asking to do this the hard way. They drew his blood 2 hours later at 2:30 am. And told him he would get the results in two weeks after he hired an attorney. They charged him with a DUI and set a court date for 5 weeks from now. They set a bail at $1,500 which was paid and now we are wondering if this is legal to arrest him and charge him with this. They never saw him drive except to pull back into the parking lot when he returned at the officers request. It seem that the officer was trying to get him for something, and furthermore, if the officer thought there may be a chance alcohol was involved isn't