Wow
First of all, no matter what, you are moving and he will have to eventually get used to it- even if it is hell for awhile (I know that's little help, but it's something to remember).
This week create a social story for him- take/use digital pictures if you can of the real events as they will happen. Start with the fact that you have to move (picture of current home), that you will pack everything up (picture of boxes), moving van will come (picture of a truck if possible), it will bring all your things to the new house, and then you will live in a new place (picture of new home)... that sort of thing. Go over it with him every chance you get... let him look at it and memorize it and ask you questions about it.
Another thing you can do is have him pack up his things. It will put him in control. Yes, his pile of shirts will not be folded neatly in the box but he will know that they are packed. Same with his toys. You can always sit with him while he does it if you dont trust him.
Let him decorate his boxes his own way. You can write the label together and then hand him some crayons and markers to label the box in his own way. This will show him whats in each box without being able to read. If he doesn't get too obessive about drawing pictures, you can have him draw pictures on the boxes that he would "need" (kitchen stuff ect). It will keep him busy while you pack.
The last box packed should be his favorite toys and his comfort objects (teddy bear, blanket, whatever works...). Put them in a special bag/box with him and put that in your car. Make sure he knows where they are and can check on them often to make sure they are still there. That box should be the first thing put in his room and the first thing he can unpack. He can then play with his favorite toys while you are busy unpacking or use his comfort objects if necessary.
Depending on how you think he would handle this (or if you think it would become a difficult habit to break) what about everyone having a sleepover in the master bedroom the first night as a "new house party". It might give him extra time to feel comfortable in the house.
Do not change his bed or his sheets... I'm sure you guessed that by now
Reassure him that grandma is not so close but he will still see her, that should be part of the social story probably.
The thing we crave in our disordered world is control. He is only 5 and has more trouble creating that control then an older child or someone like me so it's up to you to create those controls... things that stay the same as the entire world comes crashing down. Just remember that- make sure to have tons of favorite foods available, no new clothes... Keep as much constant as you possibly can.
I'll keep thinking, I have a paper to finish and tons to do
I hope this helped at least a little,
-K