My DH started out with an attitude like yours when we were first dating. Though I went about it in a slightly different fashion... I informed him up front when we started getting serious that homeschooling was not optional, it was mandatory. Originally he went along with the plan just to humor me, I think. He was certainly quick to voice any worries whenever they occurred to him! However, I think it helped a lot that my mom has been homeschooling my siblings for five years now, and that just through everyday conversation he has gotten to hear about my experiences in public school up to grade 10 and my homeschool experience completing grades 10-12. There are two things that have really turned his attitude around in recent months:
1)There was a commerical airing on the radio in the SLC area from about April-June advertising a special summer program designed to help your middle and high school students learn better reading skills, ways to study, and identifying how they learned etc... It was pretty much a "learn how to teach yourself what they give you in school" thing. I remember vividly sitting at the intersection of University Parkway and Avenue and hearing him say "I know that public education isn't great, but it's gone so far down hill that they now have to have programs to teach kids how to teach themselves?!? That shouldn't be something that one should sound proud of..."
2) He had to take a hardcore writing class for part of his major this summer and they spent a great deal of time on grammar, and he had a really hard time with it. I had an even worse time trying to help him, lol. Talking about it we discovered that between the two of us we had what amounted to about 2 years of formal grammar instruction, maybe. It was a real eye-opener for him just how much the public school system is failing its students both here and in Canada (he's Canadian), even though both of us went to what have been deemed excellent schools. This really made him gung-ho about homeschooling. Especially when he discovered that the class average (keeping in mind that this is supposed to be a class filled with best-of-the-best types) on the grammar exam was a 46%. We don't want our children getting to college and then being pleased (like his classmates were) with that sort of mark.
He has now gotten to the point that when I'm browsing around on here or looking at curriculum I'll catch him peeking over my shoulder and making approving noises. He's even started to ask specific questions and take an active part in planning. The other day he was talking to one of our (now ex) neighbors that were getting ready to leave for CA and he made some comment about how it sounded like they were going to a really nice place, but that he'd never even consider living in CA. When asked why, he replied without pausing "Because it's a horrible state to homeschool in."
I was so proud :)
I'm beginning to notice my current lack of sleep...but what I'm getting at is just keep it up. I didn't do anything to actively try and convince my DH, I just shared my own experiences and research as it slipped into everyday conversation, and he came to realize (a lot quicker than I expected, actually) that HS'ing would be a really good thing for our family and that it would be the best thing for our kids.