well, I haven't personally done this, but I've seen it done and it's fun.
Try a scavenger hunt!
Like, tell him there's a surprise, and write clues on pieces of paper and hide them all over the house.
You can also trick him into thinking he's going somewhere else, like a relative's house farther away for a week or so, so you can pack his clothes without suspicion.
As for WHEN to tell him, you can tell him whenever you'd like. However, I've always found it better to reveal the surprise when you're either on your way or when you're there, so they get the ultimate shock. I've seen countless videos on
youtube of kids being surprised a trip to WDW the-day-of, and they're just at a loss for words. I won't tell you exactly what to do, since he is your DS, and you know him best, so put your own spin on it. These are just ideas to throw around!

Make disney-shaped foods and stuff to hint it to him, make a big box a present that, when he opens it, has tons of mickey balloons and confetti, and it says in huge letters "YOU'RE GOING TO DISNEY WORLD!!!". Maybe even make a video that starts out with one of his favorite tv shows, and play it on your computer, and then let it surprise him on the screen that he's going to Disney! (not a sudden flash that says it though...make it subtle, like subtitles on the bottom or something

) It might be even more great if you personalize it, by telling him through things he likes and watches.
Like I said, there's A LOT of ways to tell him. Tell him when you think is best. Either way, he'll love it there. It's just awesome!
