Separatist yes. I wouldn't go as far as calling him a terrorist until allegations of planned terrorist attacks are confirmed by neutral parties. While he's running an org that has voolent clashed with police, there hasn't been any proper terrorist attacks by Khalistanis this time around a la the bombing of Air India Flight 182 (and attempted bombing of Flight 301) back in the day.
More accurate to just call him a violent separatist.