Subq's is like...giving them their liquids/hydrating them through their skin. So, no he wouldn't be drinking as much.

You should give the vet a call about the BM-he should be passing something. That concerns me. He may need to have the anti-diarrheal (sp?) meds cut back some..I would call the vet ASAP. On a second note, don't be afraid to call the vet about ANYTHING you have a question on-you just brought home a parvo puppy-they should expect it.