I'm assuming because you purchased your dog from a pet store that you payed good money... why would they not provide you with proper documentation/papers during purchase? Could you not return to the pet store and ask what your dog is mixed with.
Your best friend may just be a click away "www.petfinder.com"
I'm sorry Bruno, but the pet store may not know the breed at all. Have you had your dog checked by a vet? Chances are he came from a mill or a byb, and needs worming etc. If they gave you any paper work, what did it say about immunizations?
Having come from a pet store its not likely you will know for sure what the mix is, they likely do not know either. Pet shops get their pups from PM's at a cut rate price to turn around and over charge for a sick, unvetted puppy. They do not investigate the breeders they buy from, they do not care where it came from if it can make money for them.
There is a sticky at the top of the page about a test that can pin point the breeds in your mixed breed puppy, depends on how badly you want to know.
Generally the best you can do is a educated guess based on his looks.
People are like slinky's, not really good for much. But its still fun to push them down the stairs.