A.H.
Whole, uncut pumpkins should last at least a couple of months--sometimes even until spring!--if you store them in a cool, dry place. You can certainly get one now for Halloween if you can keep it uncut that long (and keep your son from trying to carry it around like my 4-year-old does! They don't last very long when dropped, lol!).
We usually wait until a few days before Halloween to get ours, but that's only because as soon as we get them, our kids start begging to carve them! :-)
Hope that helps!
--A.