My thought was... would the preschool going-away "party" make him feel uncomfortable or not? its not just about "closure" but also, maybe a young child, would not really know how to feel about it... because his parents are getting separated/divorced. And, he would not "know" how to "explain" that to everyone... but that his "Mommy" is moving. But why?
Sure, you are moving, thus he has to move schools I guess. But it is because of the separation/divorce.
And, when people ask "why" you are moving or "why" he is moving schools.... then what? How will that be answered? By you or your child?
Would you also be at that "party?"
There will be a lot of changes for him.... maybe the party might make him feel good/happy, or sad/confused.
So gauge him....
Maybe a good idea might be to create a photo album for him... of his classmates/teacher etc. Take photos and any photos you already have... and the class/teacher can write him a cute note etc.
When my friend and her family moved... that is what the class did for her son. He still treasures it even though its been 2 years ago.
Discuss it with the Teacher as well... because they have tons to do too, before spring break. Does the Teacher already know, he is leaving?
And if you are thinking of having a party for him? It would take planning...
If you are moving only 10 minutes away... does he really need to change schools?
All the best,