It's really simple, isn't it? You told him not to do it and he keeps doing it - I sense a deal breaker.
That being said, working for a start-up is no joke and the 'me time' goes out of the window, at times. When someone works for a company, it's a two way street - the developer is going to do his part for the company and make it great, but not at the cost of himself. He's also going to take care of himself.
I do not know why there's this new extremely infuriating trend of long hours in the valley and the software industry in general. A story is breaking out of the big 4 every other day in the valley. It's not OK to work for long hours. I don't know when it became the new normal.
Work is not the only thing in life, it's merely a part of it.