Viva Pinata?
By the way, he's a very lucky 6 year old to be getting a 360!
Cars.
I know it sounds crap but it's ideal for a 6 year old. It has all the characters he probably already knows from the movie, but aside from that as a game it is very child friendly. Basically unlike grown up driving games where a child would drive the car off the track straight away and not be able to get back on, this automatically puts your car back on the track after 3 seconds with barely any delay. Thus making him feel as if he is doing really well.
My 5 year old plays it on her auntie's PS2 and loves it to bits.
By the way, he's a very lucky 6 year old to be getting a 360!
Another vote for Viva Pinata and lego star wars 2.
Also my 6 year old nephew loves frogger on XBLA
Blackpool Eh, spent many a good time there in the 1980's doing an Arcade crawl from Hearts on Redbank road all the way upto Luckystar, The good old days.
Yeah the money I spent down there I could have opened up my own arcade!!
I was going to suggest Gears of War and Saints Row.
Then I realised that would be childish and irresponsible
I must be a bad parent my six year old loves GOW thinks its a blast,
and my 4 year old loves watching me play WOW on the PC he loves watching the those 40 man raids.