Outside 2/3 of MP's voting for an early GE under current legislation, I think I'm right in saying there is a way, but it is in two steps.
First, the Fixed Term Parliament Act would need to be repealed. That would require the legislation to be put forward and Parliament to debate it and then pass it. As we have seen, this can be done quite quickly and there is a large Conservative majority to pass it.
Once that is done I think we revert to the old system where it was entirely up to the PM to choose to dissolve Parliament and call for a new election.
I suspect this will happen, but possibly not until Johnson is an absolute liability and the Tories want or need to take advantage of a bounce in the polls for a new leader.
Behind the scenes the jockeying has begun and the knives are sharpening.