My American Staffy puppy is aggressive

by By Helamai
(Australia)

Hello my name is hope we bought s puppy for $1000 we even got to preorder him, we went and saw the litter when they were 1 week old and saw him and fell in love . We had photos posted to us as he grew and we put s deposit on him when he was 3 weeks and got to see him again. When we went to pick him up at 8 weeks he was very hyper. We got him home and it began.. Non stop biting lunging to bite barking growing hitting and shaking everything even my clothes I tell him no I grab his mouth and hold it shut and sat no but as soon as I let go he lunges for me again. I have smacked him and said no I have put him in a separate room and he goes nuts barking scratching to get out the door. As soon as I open it he is at it again . I'm not putting him in there for long only 5 mins so I get get my breath. He is full on. It's been 3 weeks of non stop aggression from this pup. He is nearly 12 weeks old. And it's not settling down. What do i do . I cry all the time because it's so sad ,I was looking forward to getting him I was so happy to bring home a new member of the family and looking forward to training him. I have other dogs that I have trained form 8 weeks with a little biting here and there but they were great puppies with no aggression. The other dogs are not with him he gets to play with them but not for long as they are not aggressive and he is crazy.nhe was from a litter of 11 and I don't think the owners were responsible he is so full on its Like he was rough played by humans or was neglected while he was with his brothers and sisters( the other 10 puppies) it's so stressful I'm at my wits end. My partner works from 7 am to 5 pm 5 days a week so is only really helpful on the weekend but the thing is the pup will listen to him. It will get on his bed and lay down and stay it doesn't really bite him or lunge at him or anything...why is this happenings? I feed him I'm with him all the time yet as soon as my partner walks in the house it's a completely different dog!!! He is a bit naughty with my partner but no where near as bad as he is with me.. I don't want to get rid of him he is so adorable.. But his behaviour has to stop. I can't have him with my other dogs. Please help me.