I think Sodapop is right, it is early days for him to settle in a new home, sometimes it can take months depending on the dog. He has been in a home with just him and his human and, no doubt, being allowed free reign, so it's going to be a work in progress to get him to realise he's not going to have things his own way anymore.
I agree with Nana's points, be firm when he tries to be dominant but it's also important to praise any good behaviour and check the bad straight away, they live in the 'now' it's no good scolding them in hindsight.
I wonder if he hasn't been neutered? an intact dog can be more aggressive and dominant, they calm right down after the 'snip'. Good luck with your little chap, he's probably very confused and no doubt missing his master, I'm quite convinced that they grieve just like we do.