I would say there is no hint why you have to use nominative here, so also accusative should be accepted. It could be meant like "I've seen the man." "Which man?" "This man!" - Ich habe den Mann gesehen. Welchen Mann? Diesen Mann!
Along the same line, I wrote "Welches Mann, Dieses Mann"
I wasn't sure about the dieses, but I thought the Welches was correct. So why is it changed to Welcher?
Or possibly a better question is, what does mine say?