My main concern reading this is that the business is referred to as being his, his father's, and his brother's. If you are investing so much time into it, you need to be granted ownership too. It sounds like maybe you are also investing money in it but I can't tell. He's even said that he can't do it without you, that the business relies on you, and yet their efforts are capitalizing and yours are not.
It does sound like you want to stay away from criticizing how he makes a living. I don't think his degree and his using it or not are helpful to the discussion. Not that I have full knowledge of the situation, that's just my thought from what you've written here.
Maybe the way to deal with it is really accepting, hypothetically, that the two of you are going to run "his" business as a family business. You can then talk about what hours you can commit and what wage, as owners, you will be drawing. This could mean 1. calculating the wage, which would highlight that the two of you are only earning like 2 bucks an hour, or 2. it could mean calculating the value of your time committed, e.g., if you replace a worker who would cost them $15/hour, you can calculate how much value you have added to the business. You can divide the profit by what percentage of it you are responsible for. And then you can discuss whether it makes sense financially for the family for you to give up the opportunity cost of other, potentially better-paying, work. The key here is that he sees you are emotionally committed to him and the business and the discussion is only about numbers and practicalities. But he needs to treat you like a partner and not free labor.
That would mean even more work for you. But right now you are getting all of the disadvantages of extra work and pushing back your own plans with no real benefit. This way you would at least be getting a benefit. A family business might be a really neat enterprise that could give you a lot of satisfaction--you don't mention any particular goals you want to reach though education other than moving up from dead end work, so that influences my response. I foresee that in the happy event that the business survives, ten years from now your husband will be proud of what "he" is contributing to the household while you are still earning around $2000 a month and spending 40 hours a week in dead end work, or at best going to school at that point when the return on investment is lower. Not emotionally sustainable.