I agree....patience. You've been INCREDIBLY lucky with your kids so far. It only stands to reason that every new furbaby you get increases the chances of ending up with a "problem child". He may need the tar gently kicked out of him by one of your more established kitties. I was told that all black boy kitties would prefer to be only kids however my two boys just had to "get over it" and live in relative harmony now. Hang in there kid. They will sort themselves out.
Claudia