If it is available in his music to play, then it sounds like you have purchased the regular (full length) song, instead of the ringtone. The regular (full length) song will not be available as ringtone. Do verify that you purchased the correct version from iTunes Store﹥More (…)﹥Tones﹥which features both alert tones and ringtones (each for their own purpose).
Use tones and ringtones with your iPhone - Apple Support
See your purchase history in the App Store or iTunes Store - Apple Support
The artists (and Apple) would like for you to buy these ready-to-go tones every time, but technically you can create them yourself, if you have an audio editor that can cut a segment under 40 seconds, convert to .m4a (AAC), rename to .m4r, and add to your device through iTunes on a computer.