How does his output and skills compare to the other developers? And what type of product is your startup building?
There's a wide variety of working styles that can work out for startups. Don't fall into the startup mythology of believing you have to work 7 days a week, 8-12 hours a day just for the sake of working all the time. Over the long run, you end up being less productive than a team that takes a more measured pace, because you tend to get tunnel vision and miss big opportunities in the haste to get your current goals done. Startups generally tend to be marathons, not sprints; that code you wrote at 3:00 AM in the morning early in your startup's history may end up killing it a year later when everything's been based off it and you find a flaw in the design.
OTOH, there are real cases when you need to crunch, and some startups (based on their product idea) need to crunch more than others. If you're trying to capitalize on someone else's platform that just got to market, it really behooves you to be faster than everyone else. The early iPhone and FaceBook app developers made a killing; the people who got in later, not so much. If you're commercializing academic research, this isn't as much of a factor, because there are many few competitors with the specific technical skills needed to replicate your product. Same if you're in a niche market that requires lots of specialized domain knowledge.
Regardless, you need to work out the expectations with your cofounder ASAP. If one of you expects to be working constantly and the other wants weekends off to be with his girlfriend, that's going to be a problem. It may not mean he's unsuitable for startups, but it probably means he's unsuitable for this startup. I've turned down cofounder positions because the level of intensity that the other cofounders wanted was very different from what I could sustain; that's just a bad situation waiting to happen.