Charley

Hi, I have a 12 week old cockapoo which has been with me for 4 weeks. I have noticed that he is always scratching himself so when he had his 12 week jabs I asked the vet to have a look at him as he has a red patch on his back just above his tail. It turns out that he has mites that are causing him to scratch and the vet advised me to give him another dose of Frontline. Should the Frontline work straight away or does it take time to wrok as there has been no change in his scratching? As we haven't been able to take him out due to his age and he hasn't been in contact with any other dogs could he have had the mites when we collected him from the breeders? I've read on the internet that they can be fatal so am getting a little worried.