If i was the OP.. i would have live and let live. I wouldn't have wanted to approach the situation. However, the OP has put this for discussion, and ask'd how he can toughen up. Unfortunately the focus is Tims action by talking to this man.
I think tim had every right, in the public highstreet, to say what he did. The reaction, in my opinion, tells me the man was not a pleasant man to begin with.
If it was I in that horrid situation, caught so short that you're defecating into highstreet storm drains, when approached by a member of the public i would be more than apologetic and embarrassed, and trying to explain myself, not arrogant, rude and aggressive.
Tim - how to toughen up.. Have confidence in yourself buddy. Trust your thoughts and feelings and act on them when needed. Fight or flight, sometimes flight is a better option than fight.