Ask HN: What's fair equity for cofounder without day-to-day responsibilities?

As part of a fun side project my friend as I accidentally invented a product that has become successful in a niche hardware market. My partner, who is in a more fortunate financial situation than myself, was able to work full-time on this from the beginning with little/no pay. I needed my day job so I spent nights/weekends on this. For many many months I was doing 20-30 hrs/week at the startup on top of the day job. He was working at the startup during the day as well as nights/weekends with me so he was definitely working more than me.

We’ve since rented a building and hired employees. While we're barely sustainable at this point, if I went full-time I would be making 1/3 of what I make now and I would lose all benefits. I truly LOVE my day job and have realized that I’m not passionate about working at the startup. It's successful side tangent in an arena we both know little about. I would also have no work/life balance. I've also realized that I don't work very well in business relationship dynamic that he and I have. He wants to pursue it, I no longer want day-to-day responsibility.

Obviously, a 50/50 split isn't fair any longer. We renegotiated our split based on amount of time spent working on the project-which I thought was a little dubious given the amount of time, money, hundreds of hours of design/engineering work (both our names are on the patent), and the fact that it took us both to start up. Either way, I obliged now own a little less than 10%.

He's now seeking outside investment and asked me to sell all/most of my shares back to him (at a VERY low valuation) because he thinks it’s unattractive to potential investors to see a large stakeholder who isn’t full-time.

Questions:

1. Does he have a legitimate concern about the large stakeholder issue? I feel like I've already let go of too much.

2. If so, what percentage would be considered "small enough"? I definitely don't want to throw away all my shares.

Thanks!