If one will only be able to daven maariv after chozot, he needs to say shema before chazot. What kavana does he have when he says it since after all he will say it later with brochos after chazot, i.e. should he want to fulfill the mitzvah now or when he davens maariv?
Chazal instituted that we must say kriyas shema before chatzos, and one who didn’t, is considered having disobeyed the decree of chazal. In fact if one will not be able to daven with a minyan until after chatzos he should rather daven privately without a minyan. It is controversial if the whole maariv has to be said before chaztos or not. Therefore he should definitely say shema now, and have kavana to fulfill the mitzva, and when he will daven later he will be making up what he didn’t do before.
Derech Hachaim 46, Aruch Hashulchan 135-12, Ohr Lezion 2-15-9, Ketzos Hashulchan 27-13,