My Son Got A Beautiful 7 Month German Shepard ,

My son got a beautiful 7 month German Shepard , few days later he got sick. Taking him to a vet who got testing to tell my son his distemper. The place he got the dog "Max" from said he was vaccinated one week before this could be a flash report due to the vaccination. Please tell me what's the really answer. We love Max even I just saw him once.
Sincerely
Inge Godfrey