Thanks for all the advices. It helps to know that Mocha is loved and cared for by so many of you.
Yesterday he was in pain and worn out, so he slept for most of the day. This morning, he was his old goofy self again, so full of energy and ever-so loving & trusting. I'm so glad he seems to heal well.
I chose NOT to accuse the neighbor at this time because the lack of proofs and since we're new on this neighborhood, I do not want to cause any tension. I know the best thing would be to go there and search for blood trails or anything but this neighbor is not very friendly to start with. They're rarely seen, and despite living right next door to them for the last year, I still don't know their names and how many people are living there!! They probably won't let me in.
I took this as a pricey and valuable lession for me (the saddest thing is my poor little boy had to pay for it!!). My husband double fenced the side of the backyard that is adjacent to this neighbor, and Mocha is no longer allowed to run free in our backyard without our supervision.
PS: The vet told me that this was more likely a cut wound than an accident caused by Mocha. It was straight and looked the same on all nails. It was just way too sick and heart-wrenching for me to imagine what had happened. In a way, I think I'm way more traumatized about this than Mocha... I couldn't even bring myself to look over to the neighbor's house side right now, let alone talking to them. I'd either burst into tears and couldn't speak or start yelling uncontrollably..