Fact that he is being allowed to use Downing Street as his stage tells us that Boris is still 100% behind him, and this will be an attempt to ride it out.
At this point it is manifestly clear that it is pointless seeing as Johnson as ‘behind’ Cummings or not. They are inseparable. Johnson turning against Cummings is akin to turning against himself. Whatever it is that binds them, it is insoluble.