His tattoo reads "MOTHER LOVE", which is also the name of a song দ্বারা Queen. It's progably a reference to the band as they've done it before with Claire in Code Veronica. (She wore a জ্যাকেট with the words "Let me Live" on the back; another song দ্বারা queen.)
But getting back to Billy~ Personally, I think the lyrics in the song MOTHER প্রণয় can relate to him.
