Hi C.B.,
I haven't read any of the other responses, so I apologize if I'm being repetitive.
First, your anxiety and panic are warranted. This doesn't mean you're not being supportive, you're just scared - that's normal. You need to really sit down with your husband (and possibly your parents too since you all live there and they should have a say in this as well) and explain your feelings while also honoring his feelings and letting him know you understand that he's miserable.
Second, has your husband written out a business plan? Has he done the research to know what his potential success would be in this new venture? Has he done the research to know what the start-up costs would be? Do you have the money for said start-up costs. You are living at your parent's house now, is he and they willing to wait while his business gets off the ground? Has he done the research to know how long it generally takes for this type of business to turn a profit AND for him to start getting paid from it? Will he require employees in order to start making a profit? Has he researched the legal issues surrounding this business - business license, fictiousous name; will he be a sole proprietor, LLC, S-Corp, etc. (has he researched the liability issues that go with each of these types of business entities). And, all of the other many questions and research that go into starting a business (marketing, websites, credit cards, etc.). Starting a business is much more than just hanging out your shingle and it working. You said he's a smart and college educated man, but the best of us can get really excited about something and be so miserable that we forget about the practicalities. SCORE is a great (free) resource run by retired CEOs, CFOs, etc. that help people get all of these questions answered, so you/he should really look into it.
Will he require your or anyone's else's help to get everything set up? Have you asked him how he would like you to support him?
Finally, if all of these questions have been answered to everyone's satisfaction and you're all ready to take the leap, is it possible for him to start the business while still working?
And, finally finally;) Best of luck to both of you! Whatever it is you set out to do is possible. By setting a goal and following it with practical action and perserverance, you are bound to succeed - this does not mean you may not have small failures along the way, but if you continually make smart, informed decisions, and follow your dream, everything will work out exactly as it should.
-S.