Since I decided to model this character on Elvis Presley, I began by listening to some of his early hits like "You Ain't Nothin' But a Hound Dog" and Don't Be Cruel." Then I listened and played through other 1960's Rock 'n Roll songs. I decided to write a song using a basic Blues Riff. I admit that what I came up with is not terribly original, but how many of those songs are really original anyway?
My own three sons provided quite an education for me about writing for the changing voice.
Son #1 actually sings very well and loved to sing as a child. We sang as a family and I couldn't wait for his voice to drop enough so that he could sing tenor or bass. During the time when he had only five good notes, I eased him in to part singing by having him learn alto or high tenor. The plan was to gradually have him sing music stretching into his lower notes through tenor down to bass. Then one day his voice suddenly dropped an octave. It happened so fast. (He also grew 6 inches in the space of three months.) Over night, he could suddenly sing bass. (I kind of forced him to sing bass because we needed someone to sing bass. But now as an adult, he prefers singing Tenor.)
Son #2 sang melody for a long time, but didn't like to perform in public. (He really hated the older women who came up after the show and squeezed his dimpled cheeks telling him how cute he was.) He had three older sisters who could sing Soprano, Alto, and Tenor, so he figured we could get along without him singing. So, he decided to pick up the guitar. Anyway, he prefers accompanying and he is good at it. When we do get him to sing, he sings bass.
Son #3 has a really great voice. He loved to sing as a child and was very willing to perform. Because he was constantly singing, and had developed a beautiful soprano range, he had very little trouble as his voice dropped. He could still hit the high notes in falcetto as well as sing in his natural baritone range. He did not go through the syndrome at age 12-13 of only having five good notes.
Voice teachers say, and I concur, that if you can get a boy to develop his upper range, he will have fewer problems as his voice drops. The trouble is getting the young boy to develop his high notes. Apparently tough guys don't like to sound like girls. Go figure!