That was always going to be the hard part. Thinking back to my youth the thing that got me started was martial arts. I enjoyed that, and wanted to be better at it, so I started running and doing bodyweight exercises, then got one of those cheap York Barbell sets and followed the poster inside that. Jump forwards to now and I've long ago given up the martial arts but I still enjoy the fitness aspect of it, albeit with rowing replacing running.
Of course it's easier said than done, but it might be best to try and find a sport/activity that he enjoys and go from there.
General discussions about getting and staying fit that don't relate directly to your indoor rower