I have moved several times with my kids. Something I've come to learn is that very young children have some typical fears/misunderstandings about moving:
1) that they will not be able to come with you when you move (i.e. you, the parents, are moving out, and leaving the child behind)
2) that none of the child's "things" will come with you when you move
3) that the pets may be left behind (if you have any)
In my experience, it's best to wait until a week or two before the actual moving day to tell very young children (they have no real concept of time any longer than that). However, since the cat is out of the bag, reassure your son by letting him know that your whole family (you, your husband, and your son) will be moving, all together, to a new house, and that you will bring ALL of your things (his toys, the couch, the TV, his bed, etc etc), and the dog is coming, too. Let him know it will be almost the same as it is now, just more room for him to play (or whatever the upside of the new house will be - closer to cousins, close to the park, whatever).
Since short sales take forever, just try to put his mind at ease, and then don't mention it until your moving day is set in stone!