My puppy has parvo, shaking, won't drink. Why is he acting so weak?

Poor guy. There are some complications that can arise from a pet having Parovivirus, and one of them is a condition called intussusception (folding in of the intestines). This is what I suspect may be going on with your pet Trap. He could also just have a really bad bacterial infection secondary to the parovirus and is in need of additional antibiotics. I typically put pets on Metronidazole and Cefazolin or Penicillin for total coverage. Talk to your vet about additional antibiotics. Also Trap could very well be shaking because he is in pain. Parvovirus causes sloughing of the intestines, which is uncomfortable for pets. Ask your vet about pain meds (i.e. Tramadol) for Trap. I hope this helps and I hope that he makes a full and speedy recovery. Take care.