How do you know he didn't buy the car from the thief?
We had a similar issue with the hit and run of my grandfather: even though video evidence found the car and later saw the suspect leave the car, the detectives worried a defence attorney would argue someone else may have been driving at the time the accident (e.g his wife), and therefore "beyond reasonable doubt" might be questioned.
In the end, the detectives managed to collect enough evidence to seek a conviction, and the experience taught me a lot of "unreasonable" doubts are often considered "reasonable."