Thank you all for your advice!
In the search of the information we found this great forum
Unfortunately in my country there aren't so many yorkies and there are just 2-3 breeders. Thank God there's internet for more information!
The vet told me the same think as you all did and our breeder also...I guess it's okay then if he can run that much
I always pay attention on his behaviour, don't worry
I was just adviced not to let my yorkie jump too much while he's still very young because of the patellas...I just let them jump from very low places and I take him in the arms on stairs.