I just moved from the Indiana/Michigan state line to SC! lol. You have missed the "winter" sales. They usually happen at the end of winter/beginning of spring.
Just be sure to have 2 of everything if you are going to be playing out in the snow much. Also, the schools usually require you to keep a pair of snow boots, snow pants, etc AT the school for recess, so there is no chance of forgetting to bring them. Keep that in mind when you are buying things for your son. Don't forget the warmer pajamas, also. I don't think anyone has mentioned that one. Layers are better that big heavy shirts. But, have a couple heavy things on hand for the days that temps dip below 0-10 degrees. Also, get some flannel lined jeans or some thermal underwear to put under pants.
Stock up on lotions........winters get really dry up there. Also, make sure you have a full pantry and a deep freezer also comes in handy for the occasional snow storm that creates impassible roads (it doesn't happen every year, but it can........count on at least 2-4 heavy snows per season).
Snow shovels are also a must. I always like to buy them cheap and replace them when needed. They are lighter and snow is HEAVY! ;)
I hope this helps. I will think of you while I am learning to tolerate this southern summer heat and you learn to tolerate the northern winter chill. ;)