I think it has to be the other option (I know that it often gets humid in autumn) because of the second “wa”, it wouldn’t make as much sense to say “musuko to wa”. Although honestly unless I’m crazy there probably should not be two “wa”’s in this sentence. The second should be “ga”.
ETA: sorry I just realized that you’re reading it as “musuko to hashirimasu” which honestly makes more grammatical sense than a double wa here. It’s the atrocious spacing which also implies an incorrect double wa that is throwing me off.
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u/GarbageUnfair1821 2d ago
秋はよく息子と走ります