If you're like me, you have two conflicting streams of thought... one of them is telling you that this could be amazing, your dreams come true, etc, and showing you all that it could be. The other is showing you in increasingly accurate detail, the more you think about it, all the ways it could go wrong. Technical guys are especially likely to think of all the potential problems - that's what we're good at!

My initial pattern of reaction is always very similar:

1) elation/excitement

2) Oh my god, this could all go very, very wrong

3) Maybe it can work... but only if x, y and z are resolved.

You seem to be deep in 2) at the moment. Chill, relax. Realise that anything worth doing, there's always a chance of it going wrong. If you're not failing, you're not trying hard enough. At the same time, make sure you take steps to minimise the downside to you. What if the business gets its pants sued off? You should certainly not be personally liable. You can push the terms in a direction that make it more advantageous to you. If you don't get your minimum satisfactory level to be able to get out of it without losing too many feathers even if it all goes disastrously tits-up, bail.

Lastly, the founder informed me that since he has been working on the site for years he would take the "lion's share" of the equity in the startup, and have the final say on all matters.

Lion's share is supposed to mean 100%, but I imagine you don't mean that. To get a reasonable percentage, look at the amount of work you're going to put in, and put a price on that. That's the amount of equity capital (in the form of time) you're going to put in. Bear in mind you will always get screwed if you come second and don't put in any money, but that's part of the learning game. You'll do better in the next one. Finally, as a leverage point you can point out that he needs to make your share large enough so that you don't feel that other opportunities are more interesting. You're a smart guy, presumably, and competent... there'll be many distractions. His job as a primary founder is to make sure that you don't get lulled away because he gave you too little equity. Obviously, make that point very subtly.

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.