I have a 6 month old shih tzu and he seems sick?

Hi, a friend gave me this shih tzu puppy last week and he asnt my friends but his neighbor who moved away. The previous owner didn't take care of this puppy, both his ears had mites, he has a cherry eye on his right eye and since Ive gotten him, hes been having diareah, sometimes with a bit of poop. I took him to the vet and he came out negative for parvo but they said it could be that its still incubating. I didnt leave the dog there for they were gone from 5 pm til 9 am next day anyway and I wanted to keep an eye on him, the vet recomended I give him pedialyte and ensure every 3-4 hours. He was fine, stool was fine the following day but today he had diareah with some blood again. So I will put him back on pedialyte and ensure for another week. Also this dog is very lazy, he just sleeps and eats, could it be there is something more serious? Or is it normal? He is a 6 month old shih tzu, 13 pounds. Shouldnt puppies be full of energy?