How do you go about telling your boss that you need time off to go on an Mt. Everest Base Camp Trek in Nepal?

I am planning a hike to the base camp of the world's highest mountain. I am planning to spend at least three months in that country. I have already booked an Mt. Everest Base Camp Trek too. I discussed this with my HR, but he was hesitant to grant me leave for three months straight. How do I approach it again?