I think they met at Daniel and Imogen's wedding. I have a feeling that he is Jacka's son or relative.
From other spoilers we know that his real name is Jason.
His friend run away when he sees Mark in uniform. That suggests his friend (or relative) Dustin is dodgy and Jacka is dodgy as well?
Another possible connection to someone we already know could be to Courtney. He could be her relative (brother, half brother or cousin?).