I'm sorry to hear that your dad isn't well.
My own beloved dad had pleural effusion as a result of congestive heart failure following a major heart attack. In his case, the left ventricle of his heart wasn't working properly and somehow that caused fluid to build up in the pleural space (between the lungs and chest wall). This meant that his lungs couldn't expand and so he was short of breath and uncomfortable, and couldn't lie flat without feeling like he was suffocating. It used to happen every few months, and when it did, he had to go to hospital for an outpatient procedure to remove the fluid. Basically they insert a long needle through the back into the pleural space and drain the fluid into a bottle. It takes less than five minutes, then they send you home with antibiotics. The relief for my dad was immediate.
There are a lot of causes of this, some more serious than others. From what I understand it can be a once-off bacterial infection or pnemonia that causes it, or it can be something more serious like heart failure or cancer.
You'll notice the past tense when I'm telling you about my dad, and I hesitated to respond because I understand how anxious you might be and don't want to add to that by telling you a story about someone who had this and died :( . But this isn't what he died of, not directly. He died of the heart disease that caused the pleural effusion. And my dad (who had a heart of gold) was unfortunately a two-pack-a-day smoker and would not willingly have climbed a step-ladder, never mind the Himalayas :) He died four years after his heart attack, and the heart failure (and resulting pleural effusion) occurred in the time between those two sad events.
My grandmother also had congestive heart failure, occasionally suffered from pleural effusion, and lived a good life with the condition for twenty years. She died at 89.
I really hope that your dad gets good news from his scan and is back climbing mountains soon. :)