It's interesting how differently different people view the same things. I started the Half Dome hike in my 40s (have hiked up there 15-20 times), and the Angels' Rest hike in my mid 50s. I don't know the extent of your son's maturity for such things. Anyway, my experience is this: Half Dome is obviously strenuous. Everything people are saying about the difficulty of the hike is true. The cable portion never bothered me at all. It's kind of like running a train on a track. Focus on the cables and the the 2x4s every few poles. It's a matter of concentration. I've never been frightened in the slightest, because I'm concerned with pulling myself up by my arms and pushing by my legs, not looking left and right, visualizing what might happen if I slid down the granite face (which is near impossible if I just keep doing what I'm doing). I have as considerable a fear of heights as do most people, but on this hike I can put that fear aside.
Now, as for Zion, and Angels' Rest: Zion isn't as good at safety measures as is Yosemite. I know of several hikes I would never take again. I've successfully hiked up Angels' Rest 2 or 3 times. On the last attempt I finally reached the point where I simply wasn't going to take the last part again. The drop off the left is in the 1000s of feet. The trail, as I remember it, is about two feet wide, and there is a cable on the right side of the cliff. I simply got spooked. Perhaps I'd get a muscle spasm and lose my grip on the cable. There were any number of possible scenarios. So, I did what I never do: I hiked up 95% of the way, and turned around. It's a shame. The view from the destination is wonderful--but never again for me.
There is another trail in Zion, Observation Point, taking off from the Weeping Rock area, which is one of my favorites. The entire route is safe, and feels safe. Yes you could get in trouble, but look where you are going and it's perfectly safe. The view from the top is essentially equivalent to Angels' Rest, and, to some extent, even to Half Dome. The hike isn't technical. Much of it is paved, for better or worse, much of it is spectacular on the way up, and best of all, the last portion levels off and cools down a bit. Then, voila, you're there! You'll love it. Who needs Angels' Rest?