if he can self-administer the help...then all he needs is a clock (a real physical one) placed next to pc, works well....except in situations like playing enemy territory or whatever latest thing, or waiting for closure in conversations on facebook. i've had the problem of wasting time on enemy territory. the only cure being losing interest in spending time that way. as game/application success is now often measured in addictiveness..they're developed to be addictive in the first place. in other words, another "evil" one has to fend off. introducing other interesting things to do (takes various amounts of time, money, and energy) does help to diversify their interests.
i've taken up inline skating with my kid, spent vast amounts of money on nice arts supplies/books/software etc. i think your nephew deserves (early christmas prezzie?) one of those consumer grade stylus tablets..as a mouse is horrible for any digital art.