I've been doing this with my son for years, via rerund and DVD's. He's not seen many complete runs, but he's now 15 and going through my Firefly DVDs rapidly. He also watches the modern 'fluff' like Warehouse 13.
Thinking about what shows from the past were really worth showing him just shows just how crap most of it was though.
Star Wars naturally.
TOS, TNG and DS9 plus season 4 of Enterprise.
All sorts of Dr Who.
I'm going to try him on The X-Files, Blakes 7 and Old Galactica. I think he'll like The X-Files, be O.K. with Galactica, but I don't think he will have much good to say about Blakes 7.
I wouldn't try him with crud like Lost In Space, Land Of The Giants etc. even if I owned it. I struggle with the concept that anyone watched it back in the day. I didn't...