My son has long weekend layover in Cologne this weekend starting early tomorrow thru Monday night.. I recommended the Cathedral and the Jewish Documentation center. Are there any other things to do, particularly history related, not a drinker so no brewery tours. Thanks for last.minute advice.
A day trip to Aachen by train might provide a lot of history.
The Roman Limes was going through Cologne area. The collections of the Romano-Germanic Museum might be of interest for him.
We walked across the bridge to see city and cathedral views, highly recommend, would take no more than an hour.
There is a walking tour of Cologne in the Rick Steves guidebook.
I'd suggest that your son checks when and where the (many) local football teams are playing before making day trip plans - being squished onto a train with fans after a loss can range from amusing to massively unpleasant.
The Chocolate Museum in Cologne is fun. I'd also second the Aachen day trip - plus Bonn is also a good day out (e.g. Beethoven museum; whatever the Haus der Geschichte has on while their permanent exhibition is being rejigged). There's more on museums in Bonn at https://www.museumsmeilebonn.de/