How much equity should I offer an early employee? I will not promote.

Hello, I’m running a bootstrapped fintech startup and would appreciate some perspective on how much equity to offer an early employee. I started the company in February and spent the first few months building the product with contract developers. Since then, I’ve brought on three people in more permanent roles. Two started in early September and work about 20 hours a week while finishing school. The third started in late September and works about 40 hours a week, although he is earlier in his schooling. All three are currently paid hourly. The employee working 40 hours has expressed interest in having an ownership stake, and I’m open to it. He has been central to the product’s development, consistently takes initiative, and looks for ways to improve both the product and the business. I may offer smaller equity grants to the other two, but I see him as substantially more involved. For context, I’m in my 20s and started working very young. I spent three years at a startup where I joined as the first employee. After that company was sold, I moved to another startup with the same people. I still work there full time, partly because I enjoy the work and my coworkers, and partly because my income funds this business. I plan to transition to my own company full time after we launch. Leaving a team and industry I’ve spent much of my working life in will be difficult, but this company is more closely aligned with what I want to build long term. We’re currently pre-beta and hope to launch a beta in December or January. My hesitation is mainly around how early the relationship is. He is driven and dedicated, but he’s also a student with limited work history, and we’ve only worked together for a couple of months. I suspect he’ll handle the ups and downs of startup life well, but I haven’t worked with him long enough to know how that commitment will hold up through difficult periods. School may also affect his availability over time. I recognize that 40 hours is already full time, and I don’t expect anyone to match the 80–100-hour weeks I’m used to working. My concern is about the sustainability of his involvement alongside school. I want to reward a strong employee who has been instrumental in guiding and building the company, especially while I’m not yet working on it full time. At the same time, I want to be thoughtful about giving away a meaningful ownership stake this early and how that could affect the company down the line. I also want to be fair to myself, given that I’m funding the business and hold the licenses we need to operate. What would you consider a reasonable equity range and vesting structure for someone in this position?