It’s not uncommon for kids not recognize the teaching authority that you (especially) and your husband have; he’s used to the parent this, teachers are that” separation in his mind. So, reinforcing (especially your husband needs to come on strong here) the G-D given authority that ya’ll have is vital. Communicate with him clearly that by refusing to do his work, he is disobeying G-D, too. You are his parents and teachers, it’s one and the same, and ya’ll are taking the authority over his education now, you yourselves being obedient to G-D, too. Ya’ll are responsible for your part, he’s responsible for his part.
As for the writing, he has to do copywork for 10 min, set a timer for him to hear when it goes off (not to see); if he does well, his next subject doesn’t include writing. Then, math (requires writing)-set timer for 10 min. If he does well, same as above. Keep alternating like this, include outdoor time daily.
Tell him in advance that if he refuses to do it (just sits there while the timer ticks) then tell him he will not have his free time, tv time, whatever privileges, until it is done. So, when he doesn’t do it, remove his work and give him his next subject, eventually getting back to the writing subjects.
He’s used to a lot of writing at the p/s; tell him you and his father expect him to do his best writing for only two subjects: copywork and math. Next year, ya’ll will expect more, as he will be older, but for this year, only these two are being required by you, his parents. Again, emphasizing who makes these decisions. Ya’ll do, period.
At his age, his only writing can only be copywork/penmanship and math. Everything else is oral narration and hands-on experience.
The most important thing is obedience and the recognition of ya’ll’s position; a position ya’ll didn’t have before. It’s natural that the boy would be the one challenging. That’s why your husband needs to be the heavy here, laying down the expectations and responsibilities and letting him know you – mommy – are in charge. This is vital to get laid down early because you will need it to have peace in the home and for him to have any type of educational experience and develop his character.
If you don’t get over this writing thing, it won’t get better (speaking from personal experience).
It shouldn’t take long, as long as you are consistent.
After he gets used to doing good work for those set minutes, increase it by no more than 5 minutes, if necessary,but do it without his knowing. Then, you can tell him how much time he has leftover to use for his own personal use. For ex: if he is down to writing well in 5 min, he has 5 min or 10 min to kill, as long as there was no arguing or complaining. The math, you may need to increase to 10 more minutes (total 20), if he needs it, but se the same method as above.
Hope this gives you some insight and help; this isn’t abnormal and A’s in public school doesn’t mean he’s actually learned as much as you think he may have.