Thank you so much for all your advice! Just a little update at his 7 week check up (my vet is checking on him daily and wants to see him once a week) he was 4.8lbs so he’s getting there and he is almost a completely different puppy. He’s so playful and happy! We have him on Royal Canin GI can food until his tummy gets better. They did another round of deworming and he passed some more and then I found tapeworms. I knew when I got him he had flea dirt but I couldn’t find a flea to save my life. I finally found one by his ears so we got him medicine for tapeworms. I think the passing of all the worms has messed his stomach up and his bottom seems to hurt when he poops. The vet recommended diaper cream.