If its new then you might struggle but there are a few about like at Decathlon http://www.decathlon.co.uk/rockrider-91-2013-mountain-bike-black-id_8213074.html

Or this doesn’t look too bad Carrera Banshee

Having said all that look on Pauls Cycles where they have Mongoose Salvo’s, Cannondale Rz 120’s, Cube XMS’s, Giant Anthem’s and Trance’s all around the £900 mark.

Giant Trance or the Mongoose Salvo would be my choice on the basis of versatility.

HT surely…

if i was 14 this would appeal to me….heck i’m 41 and it appeals to me!!

linky

Build one! Out of second hand parts, you can get decent full sus frames off here for around £200.

Ride IO (old Southerndownhill forum) is good for second hand stuff, but descriptions can be a bit looser on there….

Yep Rickos, don’t let the general snobbery against cheaper full suss put you off 😉

Has he ridden much before? If not, definitely a hardtail, will learn a lot more, plus as said, better for jumps, wheelies, and skids.

That Nukeproof is the dogs! #wantone

Get a second hand frame, hope wheels, fox forks and finishing kit. Then get a groupset from one of the german sites (i got an slx one for £180) and some shimano deore brakes off rose bikes.
Should work out at less than £900, my build with the above was around £700.

A medium should last 3 years, and anyway if you get a second hand frame, then if you have to sell it to get something bigger, you don’t lose much on it.

You don’t want the bike to last, you want it to fit him now. Do it right and you’ll only need to change the frame when he reaches 6 foot at 16 15 😉
It’s false economy to get a bike a kid will grow into, as he won’t enjoy riding it if it’s too big, and it’s the first few months that will seal the deal in terms of attitude to riding it. I built my lad a 16″ mmmbop for his 13th birthday, but was 5’9″ at the time. Needless to say it was ragged plenty. He’s reached 6 foot now and has swapped the frame for this:

Teaching him to swap the bits to the new frame himself was a great pleasure