I would personally start with Enterprise. It's a big jump to go from JJ's Trek to TOS. At least Enterprise looked and felt modern. May not be the best series, but should be a good introduction, and only 4 seasons. Can move on to TNG after that, and really after catching those two series, he'd enjoy any of the other ones he chose.
I would not recommend starting with DS9 or Voyager, they will be too slow and won't make much sense without watching a couple of the other series. You can try starting with TOS, but I fear for you they will seem super boring and straight up humorous and you likely won't be pulled into the Trek world.