"He is still at work" or "He still works"?
I mean to say that he has not retired yet but I'm not sure about the underlined part.

Question

"He is still at work" or "He still works"? 
I mean to say that he has not retired yet but I'm not sure about the underlined part.

Answer

He is still at work = meaning that he is in the building in which he is working just now (the speech is about his location while he works) .
He still works = meaning that he doesn't finish his work (here the speech is about that he is working and working, i.e. about his work).

Answer

at work sounds like right now he is still working. See, he still works means he's employed not yet fired.

Answer

He still works means that he's not retired yet. He is still at work means that he is phisically at his working place at this very moment.