That sounds like he's burnt out or just doesn't enjoy his work. Have you talked to him about what type of work he prefers and hates to see if perhaps you could trade tasks? I know he's your schedule, but that doesn't mean dialog can't flow both ways. You don't even have to mention the productivity issue directly, just keep it somewhat casual.
If you manage to resolve it, that'll look really good to the founder and could help the company succeed. If you fail, you still look good because you're trying to make changes to help the company.
We had an employee that was underperforming and would work the bare minimum. It turned out that he felt overwhelmed, so work was very stressful for him and he would duck out as a defense mechanism. We changed his role somewhat (internal tools dev instead of product, product manager working with partners, main handler of support issues that got past tier 1) and he was a lot happier and ended up working more hours and meeting goals.
Working at a startup/small company can be very difficult, so finding the right fit in a team can be the difference between burnout and success. If he likes what he's working on, he'll make time to get it done.