My guess is that he felt like if his own brother didn't think he was a real lawyer, then odds are that the guys at the new firm would probably not show him much respect either. Combine that with the fact that he had been trying to do the right things and just kept getting kicked in the teeth, and it at least makes a little sense as to why he would say "f*** it! I'm going rogue!" He also had the epiphany while walking in where he looked at the ring that he got from Marco and realized that what he is really good at is being a sheister.