I've been in your EXACT same situation: started a company with someone who just wasn't up to the task. In the end the co-founder will end up being the biggest burden you'll have to overcome.
It's not an easy situation to manage because even if you talk about the problem, agree on having a more evenly split workload, and even start to see some short-term improvements, in the end, things will get back to their natural state, which is the situation you're in now.
Some people just aren't entrepreneurs and have a hard time doing things, or even thinking about what needs to get done to succeed. I think at most 1% of the people are true entrepreneurs who build scalable businesses, and most people who do call themselves entrepreneurs aren't really entrepreneurs and end up building small businesses, failing or bringing others down.
So my advice to solve your dilemma is the advice I wish I had followed when I realized what you're realizing now. Get your co-founder off the project because you're not going to be able to change a person's nature at runtime. So it might mean you'll have to re-incorporate the company if you're already incorporated, may be you'll need to find some agreement to part ways, but no matter what, get him off the project while you still can because things will only get worse as the business evolves. And part ways for good: no open-ended equity deal or anything of that matter; a clear and clean break.
And that's why people say that entrepreneurship is all about execution and the people. Now that you've experienced this aspect of entrepreneurship yourself, now you know. And since you mentioned you need a co-founder, now you can look for another one knowing what to look for: people who are doers. He may not even need to be an entrepreneur himself, but for sure he must be part of the 20%-25% of people who are doers!