Test ride by yourself and two up. Another tidbit to reinforce test riding, I have a friend at one of our area BMW shops and he mentioned a couple weeks ago that he was surprised at the high % of 1600s purchased by HD, etc., riders after riding both the r and k bikes. No common denominator to share, but as much as I love twins ( be they BM, Guzzi, Triumph, etc,) the amazing torque and power from just off idle to red line on the 6 is incredible.
My last $.02
, I recently rented a HD touring bike for a few days of camping and mountain riding and if I had only ridden it on a 20-30 mile test ride I would have said thanks, but no thanks. After several days and enjoying blue skies, heavy rains and getting to know and trust the handling, I was very impressed and would def consider one for our two up travels, it has it's design limitations like they all do, but a very good motorcycle in all aspects (I do 20 +/- Ks a year for ref.).
Good luck, very cool to share the ride with your son and how great that some father/son test rides could add to the fun.