A. You're right to be worried about your husband's chucking his job at a time when the economy is slow and finding work is harder than it's ever been. If his freelance career didn't take off, he wouldn't necessarily be able to just pick up another job. And if he were lucky enough to land elsewhere, there's no guarantee he'd be any happier in his new position, since it may be that what he hates is what he does, not his place of employment.
Maybe your husband could do something to be happier, short of quitting. He could freelance on the weekends to make up for whatever he's missing at work. That might allow him to test the waters and squirrel away enough of a financial reserve to make his dream of self-employment more realistic for him (and less scary for you). He could also look for another job that would make him happier but still bring in steady pay, or take a night class to train for a different line of work. Each of these choices gives him the flexibility to figure out what he wants to do and keep his paycheck -- so you're both getting what you want. Whichever alternative he chooses, try to support him, even if it means your family will see less of him for a while.
Your husband may be talking about wanting to quit, but perhaps he really wants to know that his happiness is important to you -- that he's not just the family meal ticket or mortgage payment and that despite his responsibilities, he still has some free will. Part of your reluctance may stem from the old assumption that the man ought to be the steady breadwinner, even though times have changed, women work, and men do dishes.
Put yourself in his shoes -- would you expect your husband to let you quit your job if you said you wanted to? Probably so. Marriage doesn't mean you hand over the deed to your life -- it means you make decisions together as part of a team, with your family's and each other's best interests at heart.