So let's break this down.
Boris spends months telling us that we spend £320m a week on the EU which he'd spend on the NHS (although we don't)
Boris tells us that immigration is a massive problem (even though it's not, 0.5% of our population)
Within hours of the leave vote winning, the Leave campaign admit that not only was the £320m claim a mistake, but also that immigration isn't likely to change at all.
Boris knew it was horseshit, hence signing a letter for Cameron to stay on since he would have to deal with the fallout. Why Boris? Why wouldn't you have the courage of your convictions and do it yourself?
Cameron resigns and says feck this Boris, you sort it out. I'm not being the fall guy for your lies.
Now Boris is back tracking, "well... the margin by which we voted to leave the EU was not entirely overwhelming...."