How can a boy make his voice sound like a girl's?

Learn how to sing in falsetto

You must train your voice for singing in falsetto as explained below. A good music voice teacher can help you accomplish this and more.

It is an artificially high voice; especially : an artificially produced singing voice that overlaps and extends above the range of the full voice especially of a tenor; a singer who uses falsetto.

The trick is in the pitch and tone of the voice. Most sites cite that the female voice is just one third of an octave higher than that of a male. There are several training vocal exercises that can help but for the most part it is a thing that has to do with "speaking like a female" by means of using the same choice of wordings as a female would do. If the boy has not hit yet puberty, proper hormonal treatment will suffice to keep his voice from changing, which will mean that over time the pitch will stay put. If voice changes have already happened, then vocal training can still achieve the results needed. Vocal chords surgery is not something to be taken lightly as it could potentially risk the boy with loosing his ability to speak altogether. There are girls who have a deep voice, but their tone and their "way of speaking" (choose of words, tonal variations, etc) is what makes their voice female.

Voice is determined through your voice box, or larynx. Males usually have a thicker larynx, which means they usually have lower voices (just adding!).

Location of the Adam's Apple is key to sounding like a girl. Girls have a much higher Adam's Apple, giving them their tone. To get the sound, you must raise your Adam's Apple while keeping pitch about the same. A good way to learn this is to make your voice go high; your Adam's Apple will move up. Now try to lower your voice while keep the Adam's Apple in place. After some practice, you should be able to get a feminine voice.