The suggestion I heard yesterday was that if he goes for exit (as it appears he has), that's basically staking his future as the future leader of the Tories on winning the referendum. The reason he may have gone for that is that if the vote is for exit, then he's got that sewn up, whereas if he had supported staying in the EU, that wouldn't advance his personal ambitions, since Cameron would remain in place. In other words (and this is probably what you're saying) his decision is based purely on what it means for him, not what is best for the country.
I don't agree.
Boris has written against the EU for 20 years. Any other decision from him would have been hypocritical.