I am curious as to how hard you tried to find the original owner. Maybe someone lost him and have been looking for awhile and that's why he is skinny. Did you post on craigslist, look at lost and found, post posters around or anything. Someone may not have abandoned him he may have gotten away.
After that if you cant watch the puppy 100% of the time he should be crated or you can put a leash on him around you waste, until he gets used to your house and if you see any sniffing the floor go outside, any circling go outside. You take the puppy out from the crate at night take him outside. Puppies have very small blatters, so they need to be taken out often. Make sure you give tons of praise when they do go potty outside go crazy with praise pet , say good boy, jump around let him know that was the best thing in the world.
R.I.P- Smokey my GSD/Husky. You are my hero, my heart, my soul and you will forever remain in my heart. You were a true companion. I love You and miss You so much. Run like the wind!!