Song improved on camera for documentary, release rights issues

This past weekend, in the course of production on my documentary "RED CARPET BURN" I filmed a person at a Red Carpet event who "improved" (improvisation) an original song along with his own musical accompaniment while I was interviewing him. I got him to sign a “standard” release form, but because of music rights issues, is there anything else I could do to protect myself when I use his image and this original song in my doc so the song he pulled out of his ass doesn’t come back to bite me in my ass… The song was so funny, It might even be used in the film’s opening. The signed release covered the use of his “image, likeness and sound of my voice” but not music per say.. Thanks for the advice.