DOG IS BLEEDING FROM HIS RECTUM?

my dog's nearly 4 years old. A few days ago he stated vomiting and had diarrhea, we thought it was because we gave him chicken bones the day before, and it'd upset his stomach. Now he isn't vomiting anymore, and the diarrhea has gone, but instead when I take him out he's straining to poo and just drops of blood come out. I'm terrified it's parvo, I just called my mum and she said not to worry, as it's happened before and the vet gave him medication, but all these search results about his symptoms being life-threatening are scaring me. Could anyone suggest what it is?