We moved 4 times within 6 years when my kids were babies/toddlers. We told them we'd be moving each time as soon as we knew. My kids asked this question a lot....."Are we taking my bed? Toys? Car? Doors?_____? ______?" It was a little confusing for them as far as what was going, what wasn't. I think they were concerned we'd be not only leaving the house behind, but a lot of other things(mostly their toys!). I would just answer all their questions as they came. Is it possible to drive by the rental? Showing him the house before he sees it on moving day, might help. There's only so much a 21 month old will understand, so basically just reassuring him that you will all still be together and his stuff is going with you, will help.
Getting him set up as soon as possible, will be key to his security. We would make sure that the kids beds were set up before anything else. Try to keep his routine the same as much as possible. I know you will want to spend time getting the kitchen ready, etc. but making sure he gets his nightly bath and book or whatever your bedtime routine is will be more important than pots and pans. You can eat out for a a couple days, right?! :) Moving is stressful no matter how organized you are, so be patient. The more stressed and preoccupied you are, the more anxiety he will have. He's young, so you don't have to worry about leaving neighborhood friends, school,etc. Kids are resiliant\
Good luck!!