Kennardo included a link to the below news story in his piece. The does not confirm if the man is actually an off-duty officer, but the fact that the Chicago Police Department say they have opened an internal investigation indicates that he probably is. If they didn't think he was actually an off-duty officer, they would have used different language, at least in my experience.

Sometimes when you're writing about current events, you can only go with the best information available. Kennardo seems to have made a reasonable choice.

https://www.fox32chicago.com/news/chicago-police-investigating-off-duty-officer-allegedly-pins-boy-to-ground-park-ridge-starbucks