The first thing to pop into my head was “lacks a sense of proportion.”
Sounds like a strictly-literal thinker. Have you considered the possibility that the instructions you’re giving him are too broad? Instead of telling him “sign up for some classes,” tell him to sign up for 4 classes. Tell him that function x (or y or z in the future) does not apply to every account, and tell him the criteria that denote an account to which function x/y/z applies.
Some people need micromanagement to function. It’s up to you whether you want to micromanage to this degree, instead of looking for reasons to let him go. I’m sure you can find some.
In my world, this is my adult ADHD. The first step is awareness, which some of the above posters have been helpful with. The second step is to externalize the missing factor, such as a process chart, or a go-to person, color coding, whatever will work in this situation. Approach the problem with a team mentality. “We need to do this, but we get this. Let’s think of ways to make this work.”
This person, with the right direction, will kick ass. If you learn how to use it to your advantage, you will never regret it.