he’s still quite small for a full sized 26″ wheel jump bike for pure jumping but a jumpbike would make a good allrounder for him.
Bear in mind, almost all £400 jump bikes with sus forks will have absolutely awful (uncontrolled damping) forks that probably won’t even have aftermarket springs to set them up for his weight.
my choice for your money would be get a cheaper jump bike and scour the classifieds for a decent s/h mid range XC fork with whatever money you have left over. a DJ fork will prob be oversprung and overly heavy for a youngster.
oh.. here’s a thought.. http://www.merlincycles.co.uk/?fn=product&productId=1434&categoryId=129 & £200 left over to look for a s/h fork, mech shifter, chain and cassette