If you have a major deal in the works, be very careful about cutting him out without letting him know. There are all kinds of rules about disclosures and negotiating in "good faith". Talk with an attorney on that one because even if he couldn't "win" by challenging the situation afterwards, he might be able to make you lose.
If he's really not doing anything though, document the hell out of it and go to him with it. Lay out the case and ask him to fill in the blanks that explain his behavior. Unless he has some compelling data & reasons to back up his point, execute your plan to cut him loose.
If he has tangibly failed in his responsibilities to the point of hurting the company, "a few points" is probably too much. Buying him out might be a better option. If he's just been annoying or a pain in the ass, that might not be enough. Once again, talk to an attorney.
What do your agreements say? If you're in the US, check out SCORE.org. It's mostly retired professionals consulting on small business.
What kind of nitty gritty tasks are you talking about? I'm thinking it's possible that he drags his feet on things that he deems below his paygrade because he thinks you should be doing them, or somehow using money to solve them. E.g design, frontend coding, or similar stuff.
I hope you've thoroughly communicated your feelings to him before you decide to proceed like this.
You said he was a tech co-founder. You are another co-founder. So he's not the boss of you. You should have a frank conversation with him. It sounds to me like you let the problem drag on far too long, maybe not wanting to be confrontational? You have frontend skills and are "not allowed" to fix cosmetic changes requested by customers? It all sounds very strange.
Have you explained to the guy that from your point of view he is unacceptably unresponsive? Has he actually said these tasks are "below his pay grade" (a strange idea at a startup)?
A bit of advice: It is very difficult for business people to assess the contributions of engineers and vice versa. This story may repeat itself.