http://tinypic.com/view.php?pic=5vwqd0&s=8#.VY0aiflViko
Hi Danny, that doesn't look bad at all - it looks as though he has scraped it on something, perhaps when he was only a couple of weeks old and it's healed very quickly - if this is the case, the fur probably won't grow back. (My lab had a circular shaped scrape on his back leg and it looks similar, although his is at the front - his fur has not grown back and he is absolutely fine). It won't be a pressure sore as the tendon is going down the middle of the two patches - as the tendon is more prominent, that area would have no fur either, so he's probably scraped it a while ago. I don't think he'd be able to reach that area to nibble, so I'd rule out historic flea irritation (especially if he hasn't got fleas) and I don't think it looks like a skin condition either as it wouldn't be specifically on the back of both legs and no-where else.
See how it goes for a few weeks and if there are any changes, of course drop into your vets, but I don't think it's anything to be concerned about :-) Enjoy your pup, and all the best x