Option 1: have a meeting or a one-on-one and gently bring up the economics. Whether the MVP is actually great or not is very subjective, but you can put a number on the opportunity cost of development (in lost client revenue, using existing data). Explore some contingency scenarios: a competitor launches something similar just before you do, or someone rips off your code, or a patent trolls comes out of the woodwork, or you get hit on the head and forget all your programming skills.
You don't have to oppose your boss's strategy or goal, but if he's going to bet the company on it it's reasonable to point out what the risk factors are. A non confrontational way to do this is to ask what he would need from you if any of these (entirely plausible) situations arose, so that you are appropriately prepared. You seem like an ethical person, and asking for guidance on how to allocate your limited resources seems like a constructive approach to strategic uncertainty. You might not get answers immediately, and indeed that might be a good thing because it would show that your boss is actually thinking about the issues.
Option 2: make a power move inside the company. Ask to hire an assistant or two, train them into the less complex work, build up your own skills as an architecture/CTO sort of person. Do this in the context of a commitment to a product release, that gives your boss some security but also allows you to raise your entirely legitimate points about the time/effort involved.
Option 3: tell your boss straight out that you've hit the limit of your productivity and are rather worried about burning out due to stress. Most people would be sympathetic to the situation where you no longer enjoy your work and want to do something different, even at some inconvenience to them. Your boss would only hold it against you if you began to compete within the same space/customer base, which it does not sound like you plan to do. It's OK to have ideas and goals of your own, and it's highly likely your boss felt the same way before starting his own firm. He might well be wondering about how best to keep you interested and challenged, so as not to underuse your skills with run-of-the-mill projects.
Use your judgment about whether to have a conversation like this in the office, or maybe over dinner or some non-work environment, like a ballgame or something else you can both enjoy. Both you and your boss sound like good people, so just airing the issues may be sufficient. Just think of the situation as a bug/QA issue in the business model, and give your boss a chance to fix it the same way you would fix a technical flaw. You develop and maintain code, your boss develops and maintains deals. Your wellbeing is an important input for your productivity, and you have a mutual interest in maintaining that. Best of luck!