Aliborg - I am no longer a commuter, so I am not familiar with this man's singing.
From your original post, one thing strikes me - that noise pollution is difficult for you (and I'm sure for many people, actually).
Sadly, living in London, I don't think you will readily find a remotely affordable place to live that is peaceful, which makes me wonder if you are a newcomer to city life? As a formerly rural girl myself (Cornwall), I can fully appreciate how noisy life in the city is in comparison with rural/provincial life.
However, of all the things to find aggravating, I have to say that a busker (however good or bad he may be) would probably be low on my list of noise annoyances. I actually find the level of noise my own children make far more aggravating - largely because (also as a single mother)it's inescapable. Perhaps your journey to work is the most peaceful part of your day and perhaps the singing man rudely interrupts your reverie?!
I'm sorry you're disturbed by it. I think the human voice raised in song (however inaccurate) is the most captivating, intriguing and, mainly, beautiful sound. Is it his physical appearance that you find disturbing? If he were dressed up to perform, would you find his warbling more attractive or quirky?
I'm interested to know why you find singing tricky, you see? Do you sing yourself? I sing a lot, and anyone who's attended my Sing and Sign classes will know that I'm far from the world's best singer - but I sing with passion and enthusiasm and it makes me feel well. I think singing has a major contribution to make to individuals' mental health - and perhaps this man's singing raises his spirits in a way that nothing else can. If so, then I say to him sing up and I say to you, sing up too and release the joy of song into your life