the best advice would be to make the enclosure more secure... it sounds like you found the spot he is getting out from, now fix it so he can't if you can't fix it, then he shouldn't be out unsupervised. i hope that your feelings about your neighbor being "shady" will motivate you to do this ASAP.
getting a second dog, when you are having trouble keeping the first dog properly and safely confined would be incredibly irresponsible. additionally IF the dog's issue is separation anxiety, getting another dog is NOT a good fix for most dogs.hmmm...sounds like your dog might need a doggy friend to play with (as well as being contained completely) GET ANOTHER AUSSIE OR A COMPANION DOG.
We did that for our dog Chloe...and rescued another catahoula....Chloe and Addison are inseparable and it completely decreased any separation anxiety, helps with their exercising...and is double the joy!!!!! Some may not agree with me to get another dog...but there are a lot of dogs needing homes!!!