Tanya Prochko

Hi. Interesting question. I'm not a professional so please take my comment with a grain of understanding. I have been diagnosed with PTSD. The key word here being diagnosed. In my experience, non-professionals use PTSD as a catchall phrase for trauma response. While it IS about trauma response, it could (and sometimes does) cover a myriad of symptoms and reactions. And those symptoms can cover the range from mild to severe. But not every trauma response is PTSD.
Which is why it's important that your co-worker get an actual diagnoses. There are treatments and medications to help manage PTSD and it's symptoms. But without an official diagnosis, your co-worker won't be able to avail themselves to the available help.
No one can tell you what to expect from your co-worker until he has an official diagnosis as every case is different.
Encourage your co-worker to get an official diagnosis so he/she can get the help that they deserve.

I want to commend you for caring enough about your co-worker to want to address this with care and sensitivity for what he may be exeriencing. While there's no official diagnosis, your co-worker is sharing something vulnerable, and being a safe person to turn to is already tremendous. Some things to explore: what are the areas of his job that are affected by his symptoms? What is his expectation in disclosing this? Are there steps he's taking to address his symtoms (getting an official diagnosis, treatment, etc). The main goal is to see him as a person who's struggling at this time, which is affecting his work performance. A supportive work environment can take steps toward ensuring that he feels heard, that he receives the help he needs, and to treat him like a human being whose wellbeing is also a priority to the team, not just his productivity at work.