I get the impression that he is enthusiastic about homeschooling in general? It could be that your husband just doesn't really understand unschooling. You also don't say how young your children are. Most of us get more confident as the kids get a bit older and we have some positive feedback that unschooling is "working".
How is he undermining your USing efforts? It would be nice to know more specifics. Is it "radical unschooling" you are doing? Or unschooling more as an educational philosophy?
Unschooling is easier to embrace if a parent feels that their own success and path in life was largely self-driven, whether it involved school or not. Harder if you give credit for your success to the system you moved through.
I don't know if I'm making sense.
DH fell naturally into unschooling. DH was an artist and gardener and never felt school really worked for him. He was in his third year of college before finally seeing that what he wanted to do wasn't offered the way he wanted it. He is a professional gardener and landscaper and he is largely self-made. I did well in school, but dropped out of college to be a tripping hippie and pretty much wound up unschooling myself in the things that interested me. For us, accepting unschooling as the right path is a no-brainer. It's not a leap of faith, it is a certain confidence we have learned by making our own lives.
People who had a difficult time making school work for them are going to find unschooling easier to embrace.
For others who have worked their way clear through school and college, it would still be easier to embrace unschooling if you felt like your interests and goals were the main drivers of your education. School, especially college, offered the tools and resources to meet those goals. School is a means to an end, a useful tool to complement your internal motivation.
I think that the more credit you give the schools and external motivation for your success, the more difficult it is going to be to embrace the philosophy (especially if you believe this benefits people in general, not just you personally).
Those who find unschooling easy to embrace have a high level of trust in kids' natural-born abilities. They tend to see kids as capable, motivated, willing to learn. They tend to see that learning to focus is something they learn with time and practice, not assignments. They also tend to be a bit more patient, or perhaps it's more that they have faith that not giving writing reading and math homework early will not doom their children.
Those who find unschooling easy to embrace tend to see life as having multiple paths and ideas of success. They tend to accept that their particular path that worked for them might not work for others.
They are often comfortable with a bit more randomness (educationally, developmentally) and, yes, a bit of chaos. Anyhow, it makes it easier.
Those who put high value on having children work on stuff they don't like, doing things they don't want to do and learning to deal with that are going to have a harder time embracing unschooling. (Because achieving goals--and life itself-- involve doing things you would rather not be doing, it is best to start learning to deal with this at a young age, so goes the belief.)
If you think that learning is best proceeding in a certain order, with a particular schedule, then unschooling is not going to be as easy to accept.