As far as I understand, the result of the petition will now be sent to his managers, and then we can only cross our fingers that it makes a difference. If you haven't signed yet but also want to, the petition is still open, you can still do it here: http://www.ipetitions.com/petition/john-barrowman-concert-in-duesseldorf/
Every single name counts.
Also, for my American friends, there is a similar petition to make the dream of a concert in Las Vegas come true here: http://www.ipetitions.com/petition/john-barrowman-concert-in-las-vegas/
But please, only honest signatures, if you are really willing to travel to those places. It's our loss if we gain thousands of signatures but then nobody actually buys the tickets.