You two need to be on the same page. What that page is, I can't say, but you need to be in it together.
Time for Dad to spend a day with the child doing work at DS's actual level. (You might even tell DS that if he wants to homeschool and not repeat 1st grade, then he needs to show Dad what he can do. Make DS work for it!)
Then you and your DH need to get a bottle of wine and have a long talk alone! Hash it all out. Get on the same page.
Power struggles only happen with kids because parents make them happen. If you consciously make the choice, there's no power struggle. It sounds to me like your DH is undermining that with his comments. That can't happen without undermining your authority.
Make your best choices and stand by them. It's that simple. Then there's no power struggle.
