> He is willing to give most shares back.

I assume you had a vesting period? If so he can probably only get 25% of his shares anyway-- so him giving 75% back isn't really his choice. It was silly for you to give a large percent of the company away after doing a lot of the work yourself. Big mistake on your part.

> I argued that he still has an employee mindset. Founders don't get to quit randomly; we have a responsibility towards investors and employees who stuck with us through all this.

I don't think you're convincing him to stay. You only sound like a patronizing ass.

People can quit whenever they want. Life is too short to grind on something because, investors.