I write abominably on this site.
When I first started message boards, I wrote as if I were writing for publication (before this site). I wrote, went back and edited. Restructured. Minded my grammar. Minded the flow of paragraphs. Minded balanced content.
I wrote charming (gack, for lack of a better word) little mini articles, mini essays, that got halfway edited and sat in draft format. Finally, one day, on an ADHD board (figuring my peeps would cut me some slack) I said 'screw it' wrote from the hip in stream of consciousness, with no editing whatsoever, and just brazenly hit send. Paragraph long sentences. Atrocious flow. Dangling participles and half explored thoughts.
It was one of the scariest things ever for me (and quite frankly, I still can't blog. I keep trying.)
But (incorrect, But never starts a sentence except for in dialogue and purposeful rule breaking for effect) do you know why I write this way on here and other boards?
1) I can get away with it
2) That's what I have time for
I sell to a few UK/AU/NZ sources (far more frequently than US). Those are actual articles. I write in standard British. Anyone who has ever wondered about my mum/mom thing, is because my spell check doesn't catch it, although it does catch colour, rumour, etc. I have a very different 'voice' in those articles. I also write mediocre fiction. Short and long stories. I have a few different voices I use in those. I'm also a US student. I write academic papers in both APA and MLA styles (that's the problem with majoring in both hard and soft science AND arts).
Bottom line; I write 6 ways from Sunday. Each meeting the requirements of the medium.
Most students rise (or relax) to meet expectations. I would strongly suspect that rather than it being a problem at HOME, that the problem exists, instead, with the English classes he's taken and those whom have taught them.
My professor friends who have to grade a lot of papers are thinking about all getting together and drinking kool-aid. Writing standards have PLUMMETED in the past few years, and they're going quite mad with the quality of work their (otherwise 4.0) students are giving them. Much less the less rigorous students. English, and writing in particular, just isn't being taught well over the past 8 years.
As others have said; students are often not just bilingual, but also bi-dialect. Myself, having lived NW, SW, SE, MidWest, UK, etc... I can speak in about 11 or 12 different 'dialects' and 'accents' of American English alone.
The only way mom can be blamed, is if she's homeschooling and not farming out language arts. Otherwise, kids actually DO learn what and how they're taught.