When I was 8 year old my parents got me a remote controlled car (and I think those toys were rather expensive back then) and it broke on the very first day
wheel broke when it hit the road/pavement edge. Lots of other toys (expensive or not) met a similar fate in my hands.
If your son loves to read, that's great, and you should support him - get him a reader but before you give it to him, make sure he knows that it is expensive, not a toy, to be handled with care, and that he won't get a new one if he breaks it (unless the money comes out of his own pocket money).
The only danger I see with the Kindle is that there's a store built in so depending on how you set it up, he could end up spending money with it, or download books not suitable for 8 year olds. So a cheaper reader without Wifi could be better as you could choose which books to put on it (unless of course he already has & knows how to use a PC). But you'd have to be able to get DRM-free books for him as Kindle books don't work on non-kindle readers. If all you had so far was Kindle and you bought all your books from the Kindle store, you might want to stick to it if you don't want to deal with the world of non-Kindle ebook DRM...