According to some of the answers given to this question, it is possible that Boris Johnson could try to call an early UK general election by ordinary law, through an act of Parliament, which would not require approval by a 2/3 majority of MPs.
One of the downsides suggested for that strategy was that his political opponents might try to force amendments to the bill that would be undesirable for the PM, such as requiring him to request an extension to Brexit from the EU.
However, given that at this point he has already failed to prevent a bill being passed that would force a Brexit extension, why would he not attempt to call an election by ordinary law at this point? What does he have to lose?
Especially as it seems likely (according to this article) that he would easily achieve a simple majority (on September 4th, 298 MPs voted for an early election and only 56 voted against).