If you think this might be something he'd want to see, there are the two old trolley cars on display inside Boylston station. Not sure if there is any information about them posted there, but it might be cool just to see what the T used to look like back in the day.
Another idea might be to contact the MBTA or Amtrak to see if it might be possible to do some sort of tour of one of their facilities in/near Boston. I believe I've heard of people being given tours of some of the "hidden" tunnels in Boston if I remember correctly.