More than anything else....a correct fit. The wrong sized bike will always be a pig, no matter how fancy. I would prioritise:
Frame-most modern frames will take a lit of hammer, don't go for something heavy duty unless he's planning big jumps.
Forks-good rigid forks are better than cheap suspension. Yes, they may be harsh, but are more predictable than pogo forks.
Wheels-the best spec. you can get for the money.
I wouldn't get too hung up on running gear, most modern groupsets ride very well, and will all wear out.
Recommendations? Boardman for value, but variable levels of Halfords service. On-one bikes for decent specs and a very good ride, but a bit heavy.