I have been in startups (bootstrapped and venture) for 12 years. I've seen this movie before. People like this hold themselves in the highest regard, are not collaborative, and are delusional. Truly. I've seen this repeat itself other companies as well. The symptoms sometimes vary, but the disease is the same. This person only cares about themselves - they are a tyrant, not a partner. And usually an ineffectual tyrant, in the sense of moving the business forward.
So my advice: either negotiate your exit, or fight for your equity (hopefully you have proof), and be open with both partners about your concerns. The best outcome is you and Michael stay on, and Austin leaves. All good tech startups have three components, Business, Tech and UX, but those duties can be shared between two, and reinforced with hires instead of partners.