I'm a grandma of four now....but I remember when my son was at that age, and he also hated school. We moved, and the teacher (and the interest she took in him) really did make a difference. Hopefully, this new school will be a fresh start for your son, also.
We did have him tested by a psychologist for ADD without hyperactivity (because we knew he wasn't hyperactive) .... but the psychologist told us that he was just "all boy" and didn't like school....it was that simple.
(by way of encouragement, he's now working on a graduate degree in Geology!)
Do talk with the teacher privately ahead of time, and let her know that you're a parent who wants to work with her (so many parents are so stressed out or busy that they don't have time to work much with teachers....) If you guys can set it up so that your son enjoys this new classroom experience, maybe he'll do better. Hopefully, he can also view this as a new beginning.
I know that your time is at a premium with your job and the other kids, too......so don't beat yourself up when you don't have the time to spend a lot of one-on-one with him on school work....but some of the ideas that folks have shared, about using objects (food, etc.) to do math problems are really good ideas.
Also, if you guys are in a church or something, you might look around for a teenage boy (a positive role model) who would like to come over (for babysitter's wages) and spend a couple of hours with your son each week.....first working on school work, and then spending some time outside, shooting baskets or something....after the school work is completed.
I know several other people have suggested professional tutoring...and that is great, if you can afford it....but it might be that having an older boy (or even a teen girl, if you can't find a boy) that he looks up to, who would take some time with him....and who would let him know that it's VERY COOL when he does his best and completes his school work.....would also make a difference for him. (And it wouldn't be as expensive as a professional tutor.)
He could also save the best school papers in a folder to show to his dad when he gets home after a long time away. (And make sure your husband knows that no matter what's wrong with them....if they've been completed, then they need to be celebrated!!!)
It's a tough time....I remember. But the main thing is that he knows you love him just as he is....no matter what....and you want him to do well in school, not to just please you...but to make things better for himself.
I'll say a prayer for your strength and encouragement....and for your husband, too.