When I was eight I loved every episode of it I saw. I had a die-cast metal Enterprise D and I'd pretend it was the original Enterprise. The fact Kirk wasn't bald, liked to fight, said sarcastic comments, and didn't have gray hair made him so much cooler than Picard in my eight year old mind.
Of course who knows what amazing technological wonders your kid has already been exposed to. In those days my family had a 20 inch TV with a fuzzy picture (but unlike my cousins TV, it didn't make everything purple tinted, so I thought it was a great TV), we picked up a total of five TV stations including PBS (which was the majority of what I watched when I was eight), Saturday morning cartoons were a pretty miraculous thing more than worth the effort of waking yourself up at 6am for, and my sister's Nintendo Entertainment System was an amazing technological wonder that I really longed to take a part to see how it worked (and eventually in a very well planned out secret operation, I did).
In other words, your kid is probably already spoiled with technology to the point where the novelty of the original Star Trek, even at its finest moments, will be entirely lost on him. Daddy worship and a strong desire to like the things your dad likes are probably your best chances. Honestly, how can even TNG hold up to stuff like The Incredibles or Toy Story and whatever else they are weened on these days in the mind of an eight year old? Boring old classics like Moby Dick and War of the Worlds are among my favorite books (just to prove I am not a textbook case of ADHD or anything like that), and even I have a hard time sitting through an episode of TOS these days.
The Voyage Home is the first Star Trek film I have a memory of seeing (younger than eight), and my first introduction to the classic crew; as a kid I thought it was hilarious and really enjoyable. I didn't get why Spock was acting weird though, I figured he was just like that. Sometime later when my dad let me watch the much more violent Wrath of Khan, I couldn't believe they <SPOILERS> killed Spock at the end <END SPOILERS> and begged my dad to let me watch the next film, which he didn't own and had to track down for me. He hated V so I didn't see that until a long time after it came out, but I remember he took me to see VI two or three times in the theater some years later (I remember thinking "Oh no, not again!" the third time, but didn't want to disappoint him and was still excited about the idea of going to the movies with my dad. The Hunt for Red October was another film I went with him to see at least three times that kind of bored me after the first).
