If your co-founder isn't taking a professional approach or at least coding his way out of his other shortcomings then this may not work. If you don't have a business, then you just have a hobby.
When I work on client projects, among the first things I have to do is break down the project so that I can provide an accurate as possible quote as well as the timeline.
When the client accepts the quote, that breakdown serves as roadmap for the client.I need to start out with X and that portion will take X amount of time. After that, I go to the next item on the roadmap. My client may not know anything about development, but the roadmap is easy to follow.
If I run into problems with X, then I communicate those problems with the client and then I adjust the roadmap. With this communication and clear roadmap, the client is generally at ease, even when problems appear. The client can see where we are at, what we have done and what's left to do. The client also knows the approximate time left on the project due to my estimates.
Some projects aren't that simple, especially if you don't have a client to work with. You might be working on something which has an open deadline and which may pivot at any point. The developers of one of the paid tools that I use refuse to provide a roadmap to customers.
However, your developer can at least provide a rough sketch that both of you can follow. If you are still having problems, then cut it out into smaller chunks. Perhaps at this point you might see that your project is too ambitious and that you should cut down on the features so that you can get out that prototype faster.
Wasting time on things that aren't important in the beginning is a great way to procrastinate. The developer mind wants to work on what it feels like working on, not always what it really needs to be working on. If you don't even have a prototype and your developer is looking into "more scalable options" then that's a classic symptom of procrastination. Again, get a sketch of the roadmap. If it's not the current item on the roadmap (or in some cases it might not even be on the roadmap) then don't bother with it.
It's easy to fall into the time wasting trap. I have a start-up idea that I have been doing a bit of work on. The first thing my developer brain wanted to do was to jump into coding. What I really needed to be doing was building a very basic portal, reaching out to potential users and working on content. At that point, I was a ways from having to do any coding, yet I was spending time looking at what platform I should build it on. Time waster!
I would give this another shot. If you now the developer is good, then maybe there is something holding that developer up. Talk it through and see if you can sort of start over with some sort of roadmap. Maybe the developer is stuck on something he is having a hard time with. You really won't know anything without better communication.
If neither of you are drawing payments from this, then it's possible the project is just really low on his list of priorities. Maybe he would rather be doing just about anything other than working on your start-up idea. If the other half isn't passionate enough to be hitting this like a starving dog hitting a steak, then this probably won't work out. It's hard to find people who are as passionate about your idea as you are. If that's the case you either need to find something to pay, keep looking for another co-founder or just learn how to code it yourself. In the past three months, you could have learned a lot.