My Madison's back sort of curves too. I noticed it as soon as he came to my house (he was 2 yrs old then and an abandoned dog, so I don't know anything about his "family"). I've read where it's a genetic defect called "roachback" (which is not a very attractive term, but I suppose it's fairly descriptive
).
I wish I had a better photo of Madison's but this is the best one I could find that just barely shows it. His is less pronounced than your pup's but I can definitely tell the difference between his back and Max's.
It doesn't cause him any problems whatsoever and I've had him a little over a year.