Well, I did the same move a year ago and am preparing to move back to France, so here's what our experience was like....
I wouldn't say too much to your DC until you start packing things. Or if the movers do all the packing, tell your DC 2-3 days beforehand. Then explain that you are going to live in a new place, and you are taking all the furniture, toys, books, etc with you.
If you are leaving before the movers come, I would start bringing it up the same way you would before your summer visits, maybe a week or two ahead of time.
Keep the explanations short and simple. One step at a time! I would probably say, "We're going to Nanna and Poppi's soon! Won't that be fun! And while we're there, your little brother /sister will arrive." Don't get into the second move now, it's too abstract.
Since it takes 6ish weeks for things to arrive, I brought along in our luggage a few favorite books, stuffed animals and toys, as well as familiar plates, cups, and utensils, also his pillow and some blankets/quilts, so he'd have some familiar objects around him.
As for explaining things, I'm telling DS that we're going to France, we're going to live in a new apartment, just me, him, his Baba and our cat. I talk to him about our friends there and what we'll do. I don't talk about how he won't be living with his grandparents any more or see his local friends, I feel like that will stress him out and we can address it when he asks about it.
At 29 months he totally understands that people live in different places, which is why we don't get to see them every day. I'm sure your child will astound you by his adaptability.