Boris has no deeply-held convictions so will go for whatever is most expedient at the time/maximises his chances of retaining power.
So to be London Mayor, he was a metropolitan internationalist
To rise in the Conservatives, he was for Brexit
To be party leader, he went full on ERG, no deal Brexit
To win the election, he ditched no deal Brexit, ditched the red line of no border down the Irish Sea and accepted the EU’s opening offer which he sold as a victory. He then did not push the withdrawal deal through Parliament when he had the votes, so he could make it a Brexit election
The question now is, what form of Brexit will allow him to maintain power. The people of the midlands will not vote for him if he does a Trump Brexit and the car industries disappear. So my bet is that this time next year, he will ditch the ERG and sign up to something like a customs union/May’s deal