If you don't think you could convince him to cooperate then you still defect because he will, and if you cooperate 0 people are saved. Cooperating generates either 0 or 2 billion saved, defecting generates either 1 or 3 billion saved. Defect is clearly the better option.

If you were going to play 100 rounds for 10 or 20 million lives each, cooperate by all means. But in a single round PD defect is the winning choice (assuming the payout is all that matters to you; if your utility function cares about the other persons feelings towards you after the choice, cooperate can become the highest utility)