I'm so sorry that your new pal, Milo, took a hit from that nasty dog. Sadly, unleashed dogs are the rule, it seems, not the exception.
Be sure to press charges against the owner. I've been in animal rescue for over 15 years and believe that most dogs deserve a chance, but this mastiff obviously has serious issues and either needs an owner who can work with him or needs to be put down.
I have two chihuahua/terrier mixes and a dachshund/tibetan spaniel mix right now, all rescued from various shelters. I just love them. They're such a huge part of our family.
Sending wet licks and get well butt sniffs from my pack to yours.