As for not wanting his business to fail, I agree taking intentional actions to cause harm would be bad, but I suspect they'll cause themselves enough harm simply by you not being there soon after you leave.
From the other side of the table, I employed a handful of people in the early 2000s. We had a couple good years, and then a couple bad years. Got the point where the money wasn't coming in regularly, and I had some people stay well past the point where it was good for their own financial well-being. While I appreciated the loyalty (whether to me or solely to the clients, I'm not 100% sure) but I did tell them to leave if they could find something else, or take side work, or something.
I would never want to be in a position where I was doing what's happening to you to someone else. I've been in situations where I've been exploited, but not to this level, and it sucked, and I got the heck out as fast as possible once the reality set in.
I would suggest expanding the freelancing. You don't have much time right now cause you're working 50+ hours per week for someone else. It should not be hard to replace what is effectively a $10/hour job with some extra freelancing work. I've had a friend of mine recently jump from 'job' to fulltime freelance, and while he's not booked up 100% of the time, he's increased that hourly rate, so just to keep up with where he was before he doesn't need to bill full time.
Go back to your current freelancing clients, and explain that you'll be on the market soon. Ask if they have any extra work they need doing, ask them for referrals to other colleagues they may have who need what you do, and then ask them to be a reference. Preferably something in writing - a predone text file that you can use on your website and to include in your email marketing for new work.
Update your linkedin.com page, and hit up your network there for more work.
Finding work, especially as a freelancer, is very much about your network, and you have to curate that aspect of your life with intention and purpose. Happenstance and serendipity are great - they've brought some great things in to my life - but it's not enough. Hope is not a strategy. :) Start (re)developing your professional network outside of your current employment situation, and watch your freelancing grow.
Maybe freelancing only tides you over for a few months, and you manage to land another f/t job. Don't ever forget that you're still in charge of your own career and life. Your current situation should help drive that home for a long time, but don't dwell on it. Use it as motivation to build your professional network so that you never have all your eggs in someone else's basket again. Own multiple baskets. Even if you don't put eggs in any for long stretches, they're still your basket.
Best of luck to you, and I hope things turn around very quickly for you.