This is actually a really good question, especially considering the current economic climate.  One thing you have to realize before you put your husband's livelihood on the chopping block is that men have huge egos and also a lot of pride.  Pride gets us into more unnecessary conflicts than we'd like.  So its all in the approach.

But before we get to the proper approach let's look at a few facts.

1)  He already knows its tanking.  Despite all of his denials and behavior to the contrary, he knows its struggling, he's just trying to find the silver lining. 

2)  He also knows he needs a career change because he's probably checking his books more frequently than he actually looks at you. 

3)  Pride is a helluva drug. 

As long as you keep these three facts in mind, the approach can be quite simple.  You have to essentially cater to the same part of his ego that you cater to when you want something.  Attack with a whimper and let him know how you feel about the situation.  Don't say, your business is going nowhere fast.  Say, "I don't want you to be stressed unnecessarily and I need you to realize that trying as hard as you have is great and I think you're the greatest husband ever.  If the business doesn't work.  That's fine.  We'll come up with something else.  But don't stress yourself out worrying about being the 'big man' because you'll always be the 'big man' in my eyes!  Realize you have options and now is as good a time to look at them as any!"

Just cater to his ego and all will be well that ends well. And if not, just tell him, "baby it's not you, it's them."

We like that one just as much.