Second this. My son loved these when he was that age, and still does. Portal 2 is maybe pushing it in terms of suitability, but not too bad. I generally stick to age ratings as a rule. Portal 1 plus the extra levels definitely.
Portal 2 is absolutely worth throwing at an eight year old based entirely on how cute it would be to hear him running around the house quoting something about combustible lemons, and can you imagine a child's reaction to "How are you holding up? because I'M A POTATO."?