silky terrier aggression

Question
I have a 1 1/2 year old male silky terrier, Arnold.  I have had him since he was 6 months old and he is familiar with my family.  For the last few months Arnold has been getting very aggressive with my dad (barking, chasing, nipping).  The odd thing is that he only engages in this behavior if there is someone else around when my dad is around.  My dad has said several times that he and Arnold are best friends when it is just he and Arnold, but the minute another person enters the home Arnold gets aggressive with my dad.  I'm not sure why or how to change this.  He is aggressive with other dogs (barking - I haven't let him get close enough to see if he would bite), and he goes nuts when other new people are in the house - he doesn't bite but he gets very excitable and completely out of control so much so that I worry he may end up nipping someone just because he is so excited.  He does seem to enjoy being around people, but he just jumps and climbs and is generally very excited.  He is such a sweet dog and is loving, and I just want everyone else to see that and not let him get a bad rap.

Answer
It sounds as though he has had little obedience training and could use some.  Terriers can often be controlling and bossy if the humans are not in charge of things by being able to cue him to sit, lie down, stay, leave it, come when called, etc. whenever they want.  I don't really know the dynamic between your dad and the dog, but from what you say, it seems as though Arnold may be trying to control his movements, and that dad is not able to issue such cues and have them matter to Arnold. The solution to that is in training the dog, and your dad should participate in classes right along with you.  If you cannot afford class, or there's no positive trainer nearby, there are some free lessons at www.clickertraining.com that you can try.
Google on NILIF - that's one way to deal with a bossy dog, and you don't have to get nasty with him.  All you need do is control his access to the things he wants by making him perform a request first.

A dog that is aggressive with other dogs while on leash may just be frustrated, or fearful, or truly aggressive.  A trainer or behaviorist can help you sort this out.  Good book on this: "Click to Calm" by Emma Parsons.  In fact, her training protocol makes sense for most dogs.
Is Arnold neutered?  If he isn't, he should be. Dog aggression is usually greatly reduced by neutering in males.
His behavior, at this age, could also be indicative of his desire to assert his position in his social group. So, the key will be to increase his respect for humans:-))