This sounds very morbid, but puppies/dogs with Parvo have a distinctly strange scent. I would get him tested for Parvo ASAP, just to be safe! Puppies who spend a lot of time outdoors unvaccinated tend to get Parvo very easily.

meh.. besides that sad note, good luck with your pups! Raising 2 together is a tough job ~ I do not envy you!! Congrats on the new puppy, and good job rescuing him. If he is part Border Collie, be prepared for a high energy, demanding, and very loyal companion for life