Ask HN: Friend turned Co-founder dilemma

Dear HN 2.5 years ago a good pilot friend of mine came to me with an idea for a business in the aviation industry. We founded a company together 50/50. Now 2.5 years later we have around 5000$ per month in revenue and 7 customers. He is taking care of sales and customer support. I'm taking care of development. The problem is that I don't consider him good enough and feel that I could do a better job. His communication skills are bad, lots of spelling errors, badly formatted e-mails and subpar english skills. This is a problem since most of our customers are international. No technical skills. He has a hard time setting up his mail client. Lack of focus when working. Poor sales skills. The trouble is that when we started the company, I was very forgiving when he did stupid things, I just helped him. It has become very frustrating that he needs so much help. Not because he isn't working hard, often he works more hours than me. Lately I've started not to care about his bad work, and I tell him just to send e-mails, without me looking trough them. I've tried multiple things to help the situation. 1. I told him that I was tired of having a business with him and that I wanted to buy him out or he could buy me out. He didn't want me to buy him out and he couldn't find another technical founder. So he didn't want to buy me out either. I gave him a pretty good deal the person buying the other out, should over the next 12 months give 2500$ to the other. 2. We found an investor/co-founder, who should help him with sales. Unfortunately it didn't work out. 3. Obviously tried to be his training wheels, but without success. 4. Given him books about sales/communication, he hasn't read them. I feel like we could get 1-2 customers extra per month with the right effort. What would you do in my situation? Quit? Try even harder to persuade him to being bought out or buying me out? Feel free to ask for any additional questions.