I agree with everyone that has said that some of this is because of changes in his life. But part of those changes are simply that he's growing up. For a 20 month old to have the verbal skills to tell you to shut up, that tells me he's bright. That's a great thing. But I've seen for the last 20 years that the bright ones are also a bit more difficult, bull headed and determined to get their way. If you handle them right you can teach them to channel those leadership skills. But he isn't old enough to lead quite yet!
I have a little bit of a different approach to the shut up thing. I don't view it as a bad thing for an adult to say. If I want someone to shut up I'm going to say shut up. But that's because I know when to use it. I tell my kids that children don't speak that way to each other or to adults. I just don't believe that we can or should go through life sweet talking to kids all the time. Sometimes we just have to put our foot down. I don't want to swear at them and shut up is a strong thing to say when they are using tantrums or manipulation. It gets the point accross a lot more than being syrupy sweet to them.
I also highly disagree with telling a child that biting hurts mommy or mommy will cry. I say this because I've seen moms do this after a child punched them in the face or pulled a hand full of hair out of their head for no reason at all! It always comes up in that high pitch sqeaky voice that makes me want to scream. At this age they do not yet have a conscience so you can't appeal soley to something that is completely undeveloped. They still view us as not much more than their caretakers and the world revolves around them. Learning to even recognize others exist and actually caring about them comes in time and when we teach them to. I would firmly put them in a chair or in a crib or in another room and tell them that I don't like being around mean people and they are mean. I would make it very clear that it's wrong and bad to do. When a child is being physically agressive it is not the time to baby talk to them!
We are taught that when a child bites another child we should immediately give all of our attention to the hurt child. That way the child that did the biting will see that the hurt child is suddenly the center of attention. It will model the kind of caring that we want them to learn to have. But if they are hitting us who is going to show us this kind of concern? Only we can do that and I believe in doing it through a natural consequence. No one seeks out the company of someone that abuses them...well they shoudn't anyway. So if you give them the cold shoulder for awhile they will figure it out that treating mom or anyone else bad is going to make them rather lonely for awhile.
What happens on a playground when a child bullys others? They end up isolated because no one wants to play with them. It's all quite natural.
However, I have to say that if you are going to use these negative ways to deal with it, you have to shower them with TONS of affection when they are doing the right things.
I have raised 4 daughters, 2 are adults and one is almost an adult. The little one is the sweetest child ever and her sisters are all pretty sweet too. They have all moved into management on the jobs at very young ages. They have learned to be leaders and yet treat their employees well. My methods are rather old fashioned by todays standards.
I am NOT a fan of most of todays current "methods" for handling kids. Kids are often the sweetest, funnest, and most rewarding people to be around. But they can also be cruel. In years past people ignored this and had attitudes like "kids will be kids" That was wrong. I would NEVER allow any of my kids to be cruel to each other. I deal swift and sure with bad behavior and do my best to teach my kids how to treat each other. But, todays methods are just more of the same... The new ideas don't exactly condone the behavior. But it doesn't really teach that severe consequences exist.
I guess I'm rambling at this point. I'm tired and it's late. But mostly if you have read this far, I want you to know this is a normal age to start this behavior. 20 months can be one of the meanest ages for kids. Some kids are worse than others at this age, but they all do this stuff.
Suzi