One thing I notice about a lot of "biz" guys is that that have no idea how long some piece of work should take. If you don't know the effort, how can you possible judge if he's being productive or not? Lots of people suck at estimating. It doesn't mean they aren't productive. Maybe his top notch work is taking exactly how long it should? Maybe you should discuss a strategy where delivery is more important then code quality?
As long as you remain clueless about software development, you'll just have to take his word for it.
If you're "clueless about software development", your ONLY choice is to absolutely trust at least one software developer. No half-way about it, and if the OP can't (eventually) do this with his co-founder techie he should get out, and very possibly get out of this line of work. Otherwise at best he and his techies will be miserable.
As long as you remain clueless about software development, you'll just have to take his word for it.