Your chances of getting a puppy with a good topline with both parents having a good one were very good. But unfortunately a past ancestor must have put their two cents in
Alot of changes can happen especially before 6 months.
I wouldnt blame the breeder, not every dog turns out like we hope they will. If he had a good top line when he left Im sure the breeder thought it would stay that way, especially if your boy was older when you got him.
Maybe theres a chance the back will straighten out. How old is he?