This was a huge problem with Jaeger and Dominic. It still is, actually, but to a much lesser degree. Jaeger isn't dog aggressive, either - just dominant, like Caleb. The only time he gets upset is if another dog challenges his rank. As long as he feels that's secure, however, another dog can bite at his heels, growl, posture...whatever...and he doesn't care.
When he and Dom were first introduced, it was fine. They were fine the entire car ride home from PA. Once we got home, though, it was a totally different story. And not because Dom challenged Jaeger for the alpha male position - obviously not at 9 weeks old - but because Jaeger became very jealous of us giving any attention at ALL to Dom. He doesn't mind us giving attention to Bruno (our Corgi) or Buffy (our Pomeranian), but they were already in the family when we brought him home...
Over the past few weeks, we've had to monitor Jaeger closely. He's bitten Dom pretty solidly a couple of times, drawing a little blood once.
Every time, he's been disciplined, which has only pissed him off exponentially...
Tonight was the first breakthrough. Dom and Jaeger ACTUALLY played...pretty nicely, too. Dom started it by going over to Jaeger, who was laying down, and crawling on top of his head...and then laying down on it. It was a very odd gesture from a puppy who has been very cautious around Jaeger for a number of days now. I was quite nervous, but nothing happened. Jaeger sniffed him and they started playing. I made Jaeger stay in a down position, but they rolled around with each other, play fought, growled, showed their teeth, barked... I honestly think Jaeger had fun with Dom for the very first time. So YAY!!
I guess what I'm trying to say in my very long post (I'm good at those guys, sorry...) is that even if Caleb and this new boy don't get along right away, give them some time... I bet they get used to each other (unless this other dog is just downright aggressive).