Hi Jonny, My main criticism is that the silhouette is just not interesting enough.I can not get into your mind about exactly what you were trying to do but i can give you a run through of what i would do in exactly the same circumstances Dealing with a stranger giving you a few minutes of his time etc.
Firstly the light is coming from the left and creating a more interesting pattern on the right wall of the alley, so i would have the subject stand on the right, his foot and lower leg being diagonally in the light. To make the silhouette shape more interesting perhaps have the subject looking at his mobile phone [cliche , but you use what you have] This creates another light source perhaps giving some light to the subject in a subtle way or just as a oblong of light in his hand. There is now a lot more photographic info in the silhouette when you choose what level of black or shadows you want to apply therefore giving the viewer more possibilities to see a story.
