A couple of years ago we was looking at Halfords,
Decathlon and
GoOutdoors (Xcalibre) for my nephews.
We then dropped into an LBS who was able to offer a decent Giant (i think) for around the same price with the benefits of the LBS service and apparently more street cred.
I'd echo a bit what Ianrauk and bigjohn have said, talk to him, show him some stuff. My Nephew was quite dismissive of the better bikes and keener on the Carerra range largely because they have, apparently better kudo with his mates. I have a Carerra /voodoo clunker which often gets oohed and ahhed at, stroked and lusted over by scallys whreas riding the "better bike" barely raises an eyebrow. I sometimes think if i left the Carerra outside a shop it'd be nicked before the better bike.
What I will say is the bike my Nephew got gave him the bug, he spent about £300 upgrading it and by summer birthday he wanted a £2k+ bike. With the benefit of hindsight they have been better getting a better bike at the outset.