Agressive pitt/hound mix with separation anxiety

Question
My boyfriend and I recently got an 8 month old pitt/hound mix named Bo. He is about 60 pounds and has a very intimidating growl and bark on him. He is a big lush around the ones he loves. He is very quick to learn and already knows some tricks. Also, he will grow to be very big, therefore he must be properly trained so that he is controlable and will not hurt any one.

So heres the history on our pup: We got him from a home where he lived with five other siblings. These other dogs were very big and extremely aggressive towards each other. The owner told me the other dogs pick on him usually. Where the dogs have lots of room to run around the yard, they have never been out in public around other dogs. When the dogs growl and fight with each other, their owner (a female) would squirt them with water from a spray bottle.
When we first got Bo, he warmed up to us so quickly. You could tell he was home. He immediately began acting very protective of us. He barks and growls when someone is at the door , and he has to smell and examine each person who comes in. Some people he will growl at, and others he will fall asleep in their lap. I am concerned, however, because he has nipped a few of our friends. I am also concerned because we took him to the dog park and he showed much aggression towards small dogs. With the lifestyle my boyfriend and I live, we need Bo to be able to travel and meet new people and be able to play with lots of different dogs. My feelings toward the issue is that Bo feels like he has to play the tough guy because that is what he has been taught. My question is how do we react when Bo shows aggression toward people and other dogs? How can we teach him to be friendly to others? Should we continue to use the spray bottle technique?

Another problem we have is Bo's separation anxiety towards my boyfriend. Since Bo is part hound, he crys uncontrollably and lets out these loud howls when my boyfriend is gone. With our schedule, we need to leave Bo at home alone for a few hours a day. He disturbs the neighbors when he howls and it has become quite a problem leaving him alone. Even when I am there and my boyfriend is gone, I cannot calm him down or make him stop crying. If I take him outside he will begin sniffing for my boyfriend and following his trails. How do we overcome Bo's separation anxiety?

Sincerely,
Kim

Answer
Hello Kim!

Dog aggression is not my area of expertise, but I found this website which appears to have some really good information on the topic http://www.k9aggression.com/Aggression-Treatment/management.html

With regard to the separation anxiety, check out my blog post here:
http://blog.gizmo2wizmo.com/2009/02/03/5-tips-to-remedy-separation-anxiety-in-do...

Both of these can be very difficult to get a handle one, so be patient and consistent in your methods.  

Good luck!

Kim