So, Roughly, would this sentence just mean that he is doing work, as in an activity, while saying "Han et på arbejdet" is like, "He is at the workplace"
This means that he is at the workplace.
"Han er på arbejdet" doesn't mean anything.
I translate it to 'He's at work,' meaning that he is at his workplace and performing his job. I think it's local differences in English that are causing confusion - this is an entirely natural sentence for me to use. (native speaker)