How to train an aggressive dog?

I have a Boston Terrier. He's 6 years old and been with my family ever since he was 8 weeks. Lately, he's been getting skinny, and aggressive. He hasn't been like this in years. He did bite 2 people in the past, reason being they teased him and hit him. Ever since he bit them, he's stopped but has been in his own little world. Recently, we got a new dog. It's a girl, and she's a mastiff. At first, he wasn't too fond of her. We didn't make them meet the correct way and he immediately attacked her. I know that Boston Terriers are very serious about their territory and that's the reason why he attacked her. He hates the vet, he usually grows at the doctor, they need to put a muzzle on him. It breaks my heart cause my dad wants to get rid of him badly! What can we do to make him stop being like this? He usually steals my new puppys bones all the time! When we try to get it back from him, he growls at us like crazy and trys to bite us. He never does though, and without his bone...he's the sweetest dog ever. He gets along great with some people, he hates it when the house is crowded...that's when he gets a little crazy. What should we do?