He is twelve, has no reference points in his own history that would make him think that it was a colloquial reference to anything other than just being icky. My generation considered that word to be a borderline cuss word, but I can promise you that his generation does not. It is the same a yuck or ick or bummer.
Honestly, that statement -that his generation does not consider anything wrong with sucks - shocked me, and I have a 12-year old boy (as well as two older girls and a younger boy). They ALL know that this would be a completely inappropriate word to use (I don't think he knows the sexual implication, but just that it isn't proper English - in the same way he wouldn't write OMG-or yuck or bummer-in a written response). If any kid in my child's public school said "that story sucks" in an oral response to a teacher, let alone written, they'd receive a consequence. Now, at the high-school level, I think it is different, but I still can't imagine either of my daughters having used that kind of language to a teacher or in an essay.
My ds12 has an IEP for expressive language and isn't always the best at expressing himself (sometimes too direct, doesn't always spare people's feelings) but he does know that written answers are supposed to be thoughtful and evidence based, "I disliked the story because XX" or "My least favorite part was XX" and they should use proper English.
I know you don't want to beat him up over expressing his honest opinion, but if he has no social/spectrum issue, he is, imo, more than old enough to understand that this answer is not appropriate and to learn from you some simple rules for expresing his opinion politely.
