You have a real capability/cost problem here. From what you say about your son and his abilities with your D300, I'd say a D7000 would be the only perfect choice. Really. Really, really
BUT - the only way to get one of them for less than £350 is to go 2nd hand, which you say you don't want. Here's the issue - I went from a D3000 to a D5100 because the leap to a D7000 was too expensive. I then found that, although the D5100 was a great camera, my needs outgrew it faster than I outgrew my original D3000! So I eventually sold, scrimped & saved to get a D7100 that gives me soooo much room to grow.
If cost is king, get something in the D5xxx series, as they're great, just not a great leap up off the D3xxx cams. If you can do it though, a D7000 will keep him for years. Kudos to h for being so capable so young too - I'm jealous!