Four pregnancies here, all due dates between Sept 1 and Nov 1. My dh, who was married before he married me, fathered 9 children. 8 live births and 1 miscarriage. 8 of the 9 due dates were between Sept 1 and Nov 30.
I can't speak for the his prior relationship, but in our case I don't think there was any more "activity" in the winter. Doesn't heat decrease fertility in men?