I'm not a teacher, but since you are not getting any replies I figure I might as well.
My daughter has also had poor spelling (and grammar) particularly in primary school, although I'm not sure it was any worse than other kids. Standards are very low these days, and the teachers rarely mark spelling or grammar errors in the work as they did in the "olden days", which helped us. They also don't seem to do lists of spelling words and tests these days. I realise I'm starting to sound like an old codger but it is a worry that the kids are not developing decent spelling or grammar. ( I have also found this a big problem at work, with graduates coming through who cannot write in a way that others can comprehend.)
In primary school I tried going over her written homework and making corrections to spelling and grammar like the teachers should, but she doesn't buy into the idea of spelling and grammar needing to be correct, because the teachers don't mind, and would just end up trying to hide her homework.
I never really found a solution, but she has improved significantly anyway since the age of about 11 (she is now 13) and I'm guessing this is from reading. Certainly, people who read a lot tend to write better. She reads every day for about an hour before bed, which she is fairly happy to do (although she would often just stay on her gadgets in preference if allowed).
If you think you can get your son/daughter to read regularly, this seems like a reasonable way to deal with the problem, and probably better than trying to give him or her spelling lists and so on. It needs to be at a reasonable level, for example more like Percy Jackson type books rather than Wimpy Kid (although the latter type would presumably be better than nothing). I'm not sure if you have a son or daughter, but I've read in various places that boys often like non-fiction/hobby related reading so that might be something to try.