I am not sure about the usefulness of the phrase source singer and prefer to use "tradition bearer". In that case since he sang songs he learnt from his mother - who I am fairly sure has songs in the School of Scottish Studies archives and was regarded as a "tradition bearer" - then there is a good chance he learnt those from her.

As far his early song-writing - I feel sure Joan Littlewood mentions in her biography that "he wrote a song a day" at some point when he was married to her. But I haven't a copy of the book to hand to check.