I would definitely keep your son informed with simple words as to what is going on. Kids this age understand a lot more than we give them credit for. You can say things like "mommy is putting your toys in this box so they can go with us to the new house!" or on the day of the move "isn't this exciting?? Now we get to go to our brand new house!!" Make it seem lke the funnest thing ever. He will deal with it a lot better if he knows what's up. You could even say goodbye to the old house once you have everything out.
As for keeping him occupied and happy, there is only so much you can do. People have to move sometimes and the kids just have to deal with it. He will be fine if you have to stick him in a playpen a little more often right now. Once he gets his brand new room he will be so excited and then you can make up for lost time!