Depends how much effort you want to put in. If his parents have any sense they'll have primed him with a science book and a Nintendo DS to keep him quiet in the corner. If not, then my DS1, (9 next week!) suggests a quiz or scavenger hunt in your house (if you dare), or a good board game, like monopoly or scrabble (but obv everyone has to be available to play with him). If he's anything like our boy, he'll be obsessed with puzzles, science etc.
If you want to invest time/money in it, a 20Q gadget might keep him occupied for some considerable time, or a tough wooden puzzle, Rubiks Cube, books of difficult maths questions, mazes etc. You can also buy lots of kits suitable for this age boy, such as radio building, science stuff etc (M&S has quite a good selection). My son is overly obsessed with Yu-Gi-Oh type stuff as well but if he's maturer he might also be into WarHammer. You might wanna ask his parents about this also - if they don't mind and he's used to it, you could let him on the internet to play games - Bejewelled, Feeding Frenzy and QuicksandPyro are particular favourites in our house and if he's that kind of kid will keep him busy for hours should he be allowed!
Or for a cheaper option, you can probably find some 'household science' stuff on the internet - make a battery out of a lemon etc...
Something that involves running around if you've the space is also good for this agegroup - bouncing, jumping, kicking a football, bike ride etc...
Even old fashioned stuff like marbles or tiddlywinks is good for this age group cos they like showing off their prowess at stuff IME especially competing against adults, and can be done easily indoors if necessary...
HTH!