I think you need to either bring a friend who is comfortable with dogs or pay a dog walker to come with you.
That poor dog needs more than just having food and water dropped; he needs to go outside regularly and have some exercise also. You could also consider calling your cousin and getting permission to board his dog (he could call the boarding facility and pay with a credit card over the phone).
Honestly, very few dogs bite people. This dog is probably hyper, lonely and wanting attention and NEEDING to be let outside. So of course he is jumping on you, barking to tell you he needs things (food, water, pee break etc), and generally won't behave as nicely as he might when his owner is there and he has his needs met. Even if he has a doggie door to the outside, he needs attention, food, water and just being checked on a few times per day to make sure he is safe.