doesn't look like a pure rotti.
they're usually more compact, and this one seems a bit light and lanky.
perhaps he's mixed with Schäfi.
Rottweiler can have white blotches, that makes them being not standard enough to be part of the breeding stock.
generally you can go by the rule of thumb that a dog that doesn have papers of one of the big breeding clubs will very likely have some kind of foreign breed in the mix, or at least you can't prove that they don't have it.
which doesn't make thet dog worst or better.
mixes can be very good dogs as well as purebred. the only positive aspect you've got with a purebred is that you find out more about the health history of the ancestors of dog and the personality, working ability and look will much more like be in a certain range fitting the breed.
they're a bit more predictable in character than mixes or unregistered "purebreds".