I worked as a preschool teacher in an early intervention program. I have some experience working with children with Down Syndrome. I know language is a common issue with children who have this special need. Although this is typical behavior of a 2 year old, he is letting out his frustration because of his inability to communciate (I am assuming). Be persistant, make picture cues for him to refer to (transition cards). You can make pictures of emotions, things he wants needs, etc. I used to carry the cards on my name tag. I would have pictures of every step of the day. Down to going potty. Here is an example of how it can work:
Instead of saying time to go potty, most likely he will ignore.
1. Show the picture of the potty to the child
2. State what it means to the child.
3. Have the child repeat it back.
4. Follow through on task.
When he is frustrated make a board of emotions. Have him go to the board and point to the happy, sad, mad etc.
This sounds long and tedious but believe me it works like magic!!