@ShowOfHands I spotted your post belatedly and had to reply.
I have a PhD in English literature and am a lecturer and a published author. Feedback/advice on SPAG unfortunately forms a part of my job: a tedious part, I will admit, as I'm far more interested in content and ideas. Students can also use a proof-reading service before submitting their work (but it's amazing how many of them badly need this service and don't use it, despite constant reminders). I also a frequently editor and peer reviewer.
It would never, ever occur to me to go about 'correcting' others' SPAG on an internet message board. It's the height of supercilliousness and bad manners. I've often wondered why posters who make a habit of doing this don't apply for jobs as A' Level or GCSE examiners because goodness knows there's a shortage. It's not needed or wanted on a forum such as this: not unless the expression is so poor as to be incomprehensible (in which case I'd tactfully ignore), or the poster dropped an amusing grammatical clanger that they'd likely find funny themselves.
I've helped students who have arrived at university with undiagnosed literacy disabilities. I'm not an expert - dyspraxia for one needs formal diagnosis by a medical professional to rule out physical causes for the condition - but I can sometimes spot when a student's clear intellectual ability doesn't translate itself to paper as it should. I can then discuss with them whether they've noticed they struggle with this, and if necessary refer them for diagnostics.
My own son (9) is the one now in that predicament. He's always stuggled with spelling and I blamed the dreadful phonics system of teaching literacy and the fact that he does sometimes need a rocket up his backside to persuade him to learn spellings and do the minimal amount of homework set by his primary school. As he's entered Year 5 it's now been flagged that there is a high likelihood of dyslexia and/or dyspraxia and he's been referred for proper assessment and diagnosis (at our cost, it has to be said, owing to our woefully underfunded LEA. I'm devastated for children with disabilities whose parents can't absorb those costs, which come out at nearly £1K. I feel strongly enough about this that for the first time in my life I have drafted an email to my MP.
I wanted to lend a note of support to you as it's heartbreaking to read of how badly this is affecting your son. But before I was the Mum whose child was also in this position (it was flagged all of two weeks ago), I'd still never have mocked anyone else's SPAG. It's ugly behaviour which says far more about the 'corrector' - whose own posts are very often riddled with errors of their own - than it does the 'corrected'.