I bought a like-new Beretta 20-gauge Black Onyx with 28" barrels for my son a few years ago at a Cabela's Gun Library (pre Bass Pro) and paid $900. I flat-out got lucky.

Used Beretta, Browning/Miroku, and Japanese SKB/Ithaca/Weatherby OU's would be good choices, but you may have to do some digging at that price point. 26" barrels usually go for less money than 28" barrels if that matters to your nephew. As previously noted, the BL-series Berettas are starting to get a cult-like following in some areas. Getting parts for them also is difficult per Rich Cole. 20-gauge Ruger Red Labels weigh close to a Beretta 12-gauge 686. If he is willing to consider a 12, you can find a Beretta that is closer to his price point and weighs around 7-pounds, sometimes a bit less.

If he plans on putting only a few boxes of shells per year through it than some of the Turkish guns may suffice. If he plans on doing any target shooting I would go with one of the above brands. I know a few guys that used CZ guns during trap league and they all had issues: triggers, ejectors, and/or firing pin problems. They all moved on to other brands. CZ does good warranty work; they get lots of practice.