Even though the officer didn't do any field sobriety tests or breath test on your son, he can still be charged with DUI based on the officer's observations of his behavior and appearance (i.e., if he had slurred speech, uncoordinated actions, odor of alchol, bloodshot eyes, etc.) that he may observed during his contact with your son before sending him home after the accident.
As further proof, the officer can also use any admissions your son may have made about the state of his sobriety when he confessed that he was actually the driver. He can also use the fact of the accident itself, if it tends to be the type of accident that likely wouldn't have happened under "normal" circumstances, when the driver was sober (intoxicated drivers often take greater risks). Additionally, the officer may have obtained a statement from the passenger who was in the vehicle at the time of the accident that may incriminate your son.
It's tough for the prosecution to prove a DUI solely with this type of evidence -- a breath/blood/urine test and field sobriety tests make it easier to obtain a conviction -- but it is not impossible.
One option in this sort of situation is for your son to hire a lawyer, or, obtain a court
-appointed attorney if he can't afford a private attorney. An attorney could file a motion to dismiss based on lack of probable cause
for the officer to charge him with DUI under these circumstances -- i.e., that the prosecution has insufficent evidence to prove that he was under the influence of alcohol at the time of the accident.
Hope this helps, and good luck.