Brian not playing guitar live anymore is speculation. Ian Perkins is 100% playing with TGA on stage now, as confirmed by a twitter post. I'm too lazy to find it but look at TGA's twitter and someone asked the question and the answer was something like, "Official title is touring guitarist."
As stated by a few people earlier in this thread, they think that the band will just be a 5-piece with 3 guitarists. I can't imagine Brian just singing and jumping around, so I think this is most likely.