John remembered the dream - how could he not? - but pushed it to the back of his mind because he had more important things to worry about. It was just a dream and he had to accept that. But two years later, he started to wonder when shit started playing out exactly the way the boys said it would. He knew enough about this world to not discount something his mind cooked up but he avoided thinking about it for a long time because he couldn't handle any of it at the moment with everything else going on. Burying himself in denial was easier and had worked before. And when Dean was in the hospital dying because of Yellow Eyes, he knew what he had to do without question. That's when his denial finally broke and he accepted the validity of his dream, came to terms with the fact that he actually time traveled to the future. Thinking of how Sam will find him and that it'll haunt him for years afterwards broke his heart but he took comfort in the fact that they will get closure eventually. The boys and Mary matter more than him anyway in the long run. The fact that they'll save the world numerous times proves it.
When he escaped Hell and helped the boys take down the demon that ruined their lives, he's even more proud of them. Everything he wants to say is practically bursting out of him but they'll get to it on that unexpected, beautiful, and wonderful day in 2019. The boys have a shitshow ahead of them but John knows they'll take care of each other and Mary. Knowing what he does, what's ahead, he's at peace for the first time in a long time when he starts to fade out.