* He doesn't understand the stack well... If you can't get him to stop coding, Get him on source control and branching, but do peer reviews into master, by which I guess I mean keep that power for yourself, or, if that's contentious, keep a separate branch for yourself, and if you see bugs discuss them with your partner. Dialogue is the way to help here, unless he's willing to defer to your expertise, but good code management and practices will help, and keep him engaged. If he's set on it, he may as well contribute properly.
* Re: not seeing features in the code This is an odd one. What is the proper way for you all to discuss features? If you're not both power users or both completely familiar with the code base then how should he know about a particular feature? Is your documentation air tight? Probably best to agree to a stick-to-it answer, if he doesn't know or thinks a feature doesn't exist... this is true for all salespeople who, as you grow, will grow increasingly unaware of features. Something like "let me take that back to our developers -- if we don't have something that precisely does what you want, we can add it as a feature request" or even "they're constantly adding capability and I may have even missed it in our latest version" or any variation. Again, discuss it, though.
*re: taking out his laptop... If he's the sales guy, and this helps him sell it, then I don't see the problem. If this is costing you sales, and you're sure that's the reason, then that's a different discussion. While this isn't an approach I'd take, I'm not "salesy". Again, discussion is your best option, but if you're asking him to stay in his swim lane, then consider whether or not this is wading into his. Better you're both on the same page, I agree -- if the issue is letting clients see trade secrets then, as before, agree with your partner on a party line: "if we can get an NDA in place, maybe a developer and I can sit and walk you through the code if you have a question about a particular piece, if that will help make the sale".
re: damaging the relationship.... just talk to him about it. Raise concerns -- point out bugs and use that to justify asking him to be more diligent, or pass code by you; point out lost sales and discuss what you think may improve. Approach it as a collaborative effort to solve problems, and hopefully he will take it well.