Mom (50) bought my brother (16) a car from provate seller and cant put the car under his name

I (28 male) just found out my mom bought my little brother (16) a car to drive around.
Its a used car of course but the issue is that they never went to the department of motor vehicles (arizona) to change the previous owner name to my brothers name.

I told them to get in contact with the guy but he is nowhere to be found so the car is still under the seller’s name.
How can we go about putting the car under my brothers name