We have an 18 month old rescue collie/lurcher cross. Did puppy training, gets exercise everyday, goes to doggy day care to mix with other dogs. I have two ds 7 & 10. The dog bit my husband on the arm recently and actually broke skin, he also snaps aggressively at unknown children. Very needy in general. Am terrified he will end up biting a child. No underlying medical issue. What to do? I don't think this can be trained out of a dog?