I want to add another possibility that has not really been covered by the other answers. Perhaps hinted at, but not explicitly. Obviously, this may not apply in this particular case due to the people being unknown to me, and myself not having the professional credentials to say if it applies either. But:
People with a certain "mental makeup" are often drawn towards Software Development work, and initially this works mostly well: they function securely in the precise machine-interaction world. It starts to become difficult when they need to interact with (and perhaps exert management control over) humans and the human world of ambiguities, idiosyncrasies, unknowns and unknowables.
What I loosely term "mental makeup" above is sometimes diagnosed as (Adult) ADHD, Aspbergers', (High-functioning) Autism, or the general term Autism Spectrum Disorder, which has physical, developmental and environmental underpinnings amongst others - so not necessarily the person's "own fault". I understand that this area still generates a lot of controversy in the psychiatric profession, which along with other factors causes it to be under-recognized and under-diagnosed (from one side of the debate).
Whether or not one agrees with this being a genuine condition, I feel a number of very beneficial practices have been developed for the management of the outcomes via behavioral therapy, and a person could probably apply such if he feels they are applicable, diagnosed or not.
Just taking the general points made by the ADHD article above, intelligent people often learn coping skills into adult life, and while those coping skills are usually sufficient for a basic function (such as his current job tasks), they are not sufficient for excelling (which also explains the comfort zone). The positive point made in various articles is that given the right help, and playing to the person's strengths instead of trying to correct weaknesses, may transform the employee into an excelling asset to the company (and himself). The danger here is that the traditional corporate "career path" task/responsibility assignments, and the path of "improvement plan or else firing" will often exacerbate the problem due to the increase in stress. Also, the normal pop-culture platitudes (e.g. "challenge your comfort zone!" or "it's all in your head!", while they may be true in a literal sense, are not helpful.) Such conditions can also lead to a lot of destructive behaviour, one should acknowledge when a person is adult enough to have curbed such impulses.
Such a person often finds it difficult to "figure out the adult/corporate world" and function in it at a seniority-appropriate level (a.k.a. "adulting"). Typicals/"normal" people may not appreciate how deep this goes, and may be exasperated by the "basic" things gotten wrong that the typical takes for granted. (Friendship and mentoring with a typical with enough empathy to work through this may be beneficial.) One will also often find a lack of confidence and self-esteem, leading to a risk-aversion in taking on new challenges due to an (overblown) fear of failure. And lastly, figuring out social interactions and relationships is also a challenge, so they will probably appreciate someone taking interest in them as a person (not only as a fungible corporate asset).
Sexed terminology: You refer to your co-worker as "he" and in this case it might be relevant: about twice as many diagnosed people are male as female, and there may be some genetic link (see articles). However, it is reported that females' experiences of the condition(s) differ subtly from males - although a lot applies to both sexes. Here is a recent article about ADHD from an adult woman's perspecive which I found insightful.
A lot of helpful material may be found via websearches: "adult adhd career | work" etc. Here is the relevant section of a site I stumbled on just today, way-out country and all, after reading the above women's article. I've initially become interested in the subject due to various personal stories on IT-related discussion forums I've read over the years, shared by people in a work-related context.
If I were in your place, I'd approach the issue along the following line (incidentally, I think it may be appropriate for all subordinates, so nobody needs to feel left out):
- Spend time with subordinate(s) to get to know them on a personal level. E.g. have a one-on-one coffee/lunch break with each one, twice a month or more if practical. (Or if you are both smokers, I've seen that in the age of indoor smoking prohibitions a lot of bonding can happen at the designated smoking spot.) Just steer in a more focused direction: How do you experience ? I find sitting close to the coffee machine distracting due to all the chatter, what about you?
- As a supervisor, discuss a "career improvement" initiative with subordinate(s). If not company sanctioned, this can be informal and lo/no-budget. I'd suggest finding out each person's strengths, exploring those, and sharing out tasks according those to improve team performance.
- Some tools like the Gallup/Clifton strengths finder (paid) or 16personalities (free + extended material paid) among many others may be helpful. And I acknowledge again that these are considered not scientifically rigorous enough by some practitioners, but may still be helpful to get something going, for lack of better tools.
- At some point during the above, and knowing your coworker well enough on a friendly level to allow such a remark, you could gently steer him towards reading up on ADHD (etc.). Even something sneaky like "I read this article on ADHD today, wow, didn't know that was a thing, just imagine having to struggle through life thinking there is something wrong with you but not knowing what, and just struggling on in the old rut." If he recognizes himself in it, he can find out more and seek help. I'm not sure, but I feel that in many jurisdictions it would be illegal or at least unethical for an employer to allege a certain medical condition in an employee and demand he undergo medical treatment - has to come from the employee informing the employer if he asks for Reasonable Accommodation.
- ... Then follow through with applying the team members according to their strengths where possible; it might just be that your team, your company and yourself might benefit from it too.