Can I ask potential advisor what topics he is currently most interested in?
You could ask, but it would be much better to arm yourself with knowledge to facilitate asking a better question.  Do your homework and search for his recent publication record, and find and read the recent results.

So yes, asking "what are you doing now" will reflect somewhat badly on you.  Saying "I'm aware of all your published work, and wonder if you are looking into any new directions that aren't published yet" sounds (and is!) much better