OK well mine didn't show up until around 14 and I didn't even know there was a word for it until I was an adult, it was just something I did (still do) iyswim.
I can tell you my experience, but that's all I have because I don't know anyone else who does this and I've never had any treatment for it.
I have never fully stopped since I first started but when I first started pulling I gave myself a small bald patch. Once I realised how much hair had gone and became aware of what I was doing I was able to cut it down a lot and confine it to particular areas on my head.
Why do you think he'll move on to eyelashes? Is he showing signs of that? I've never even been tempted by eyelashes or eyebrows. It wouldn't give the right "feeling", if that makes any sense. It would hurt too much too.
It's basically a lot like nail biting I think. It's very hard to stop because sometimes you do it unconsciously, but it's not totally beyond your control (although he is only 8, so might be harder for him).
I don't bite my nails at all but I know a lot of people who do and although I can't understand how you could bite your nails down to the quick until they bleed, in another way I sort of get how you can do something compulsively that gives you a feeling that is kind of painful, but doesn't really feel like pain.
Sorry, I'm not explaining this very well.
Basically it has very little impact on my life at all. I have some patches of shorter hair around the back of my head and near my temples that I pull. It's not really very noticeable and I rarely think about it. It has had very little impact on my life overall.
The phrase on that Wikipedia article "Individuals with trichotillomania live relatively normal lives" made me LOL
Cutting his hair short sounds a good idea to me. When I go through phases where I pull a lot (usually if I'm stressed) I sometimes cut the hair I pull very short so I can't grab hold of it.
It's kind of weird telling you all this because it's stuff I barely ever think about.
If you're worried about it I would maybe mention it to your GP and see if you can get him seen by a specialist who could help him to at least minimise the effects.
Hope some of this makes some sense.