Get diagnosed. Get medications. You might find that you're working with a completely different person when that is resolved.
Meanwhile you kinda need to create more accountability for the. Have them send you reports daily or twice a day on work accomplished. Even hourly if needed. You don't even have to read them. The goal is for him to feel accountable and use that as his external motivation tool. Have him write more documentation and agree ahead of time that you set the priority.
There's a lot. Much more than will fit in this post. But a lot of it is about scaffolding his work experience to force him into a focused mental state. Do not judge him yb lines of code, only by results and consistency. And absolutely call him out to remind him.
Also tell him all this. But remember adhd people have trouble connecting knowledge and action so you'll have to remind him that this convo happened all the time which will snap him back into understanding.
ROFL: You can tell where my ADHD lies. I got about 50% of the way through thinking you were complaining about a co-founder looking for advice.
I am going to leave my reply up rather than deleting. And just to note, my ONLY productivity tool that has ever been effective is a piece of paper in front of me right by my keyboard that I can't not look at containing my notes / checklist of things I am currently working on, ordered by priority.
Definitely re-title it to "Show HN:"