If the rules should have been taught long ago, then he shouldn't have had any grammar mistakes in his writing. If he wants his paper read for content, then he should not waste the teacher's time by filling it with elementary grammar errors.
If this was English 101 and the criteria for writing the assignment was "correct punctuation and grammar", then he needs to meet the criteria and have correct punctuation and grammar. If it's any sort of "formal" writing, then there is no excuse for incorrect usage of the language. Unless it is a creative writing assignment where the teacher specifies they can get creative with their grammar, then his teenage-angst short story with incorrect grammar can be done on his own time.
I agree that a computer would not be able to grade creative writing because of allowed variations to the rules in that genre, but even in creative writing you cannot just haphazardly write with crappy grammar and then expect to get a free pass because "It's creative writing. Grammar doesn't matter!" Yes, grammar does still matter. You need to know all the rules before you can break them, and "good writing" means they were broken in deliberate and thoughtful ways. It's usually easy to tell which are "thoughtful errors" and which are just plain laziness. For every wonderful piece of creative work, there is a ton of bad paintings, bad music, bad photography, and I've seen plenty of "creative uses of grammar" that made me want to claw my eyes out, all of which can quite easily be graded as "bad". Even painting, music and photography have their own version of "grammar", and I think in many, if not most cases of "bad" art, it's a matter of the artist either not knowing the rules in the first place, or else breaking the rules through sloppiness and laziness rather than thoughtful deviation.