If you were both starting from scratch and equally investing time and/or money, 50% would be fair. However, this person has already pre-funded the startup, built the product, and handled the initial development and data. Given this investment, 50% equity for you doesn’t make sense.
From our experience with early-stage startups and various incubators and accelerators, 50% equity is only reasonable when starting from scratch. Since the founder has already invested in and developed part of the product, a fair range for you might be 20-30% depending on your overall value (e.g. beyond dev too, are you ex-airbnb, facebook etc that his investors will drool over or mid-senior dev).
The most critical aspect you need to evaluate is his ability as a CEO to sell and raise investment. 50% of nothing is pointless, but 25% of a successful venture is valuable. Assess his background, experience, and network in sales and fundraising. If he can drive the company forward, then 25% is a solid offer.
It’s also important to have a transparent conversation about your roles and expectations. The equity split should reflect the risks and contributions both of you are making.
P.S. The vesting terms are really generous indeed. Maybe ask him plainly why he set such terms compared to industry standards.
Goodluck!