I guess you could say I approach it somewhat naively hoping just the experience of being a co-founder would be worthwhile
Some people say that co-founding a company is kind of like getting married. And other people say that marrying someone based not on what they are, but on what you imagine they might become, is the height of folly.
Anyway, the problem doesn't excite you, and your co-founder's approach doesn't excite you, and your stomach has obviously figured out that this would be a mistake for you even as your brain continues to debate the issue, so it's time to discover a prior engagement and politely decline.
i'm working on something i really love, but the small parts of it that are boring are painful to slog through. i wouldn't recommend being in a startup that was nothing but that.
the work is too hard to stay motivated if you aren't super pumped about what you're building each day.
as for the stock, rather than comparing your % to his %, figure out what you would need to get an exit you're happy with after 3 years and 60% dilution from investors, and use that as your lower bound.