English Translation
He wants to become a doctor.
Why isn’t this él quiere ser médicoz?
He wants to become a doctor.
Why isn’t this él quiere ser médicoz?
He wants to become a doctor.
You can use either sentence. But if you want to emphasize the “becoming,” you need to state that he wants to become one. It’s a shift in emphasis, expressed in both the Spanish sentence and the English translation.