A particular Heinlein
Farmer In The Sky is about a boy and his family colonizing Ganymede I loved it when I was 10-11.
If he is precocious, he might like, When Worlds Collide. by Edwin Balmer and Phillip Wylie, I read it and reread it when I was 12-13.
If he is very tech savvy these might be very well too dated for him, but I think they are timeless with just a little suspension of disbelief.
There have been many Star Trek novels and I have heard they are pretty good.
Another possibility is Bertram Chandler's Captain John Grimes, Galactic Rim space operas. They are definitely right for an 8 year old. Once again, timeless stuff.
Baen EBooks a great web site for free SciFi ebooks and they have a lot of juveniles as well as adult science fiction.