Children can have an experience being bullied at school or on the bus that no adult is aware of. As someone said, children have a difficult time talking about this with parents or anyone else. It really sounds to me like something may have happened on the bus b/c the teacher said he did fine at school. Bus drivers are driving with their backs to the students with high seats in their view when they look in the mirror. They can't see everything. Bullying is a tough issue. It could be that one or more of the kids at the neighborhood playground could have been involved in the incident. It also could be that he is having trouble socializing with others. Calling things boring is a defense mechanism. He thinks if he is bored with something, you won't make him do it. Maybe you can convince him to tell you if something happened on the bus or at school with another child.
If you're dealing with separation anxiety, it will take more than one week for the butterflies to subside - months and for some children, years, But, I wonder if social anxiety might be more the issue here (not sure though). And, because a child suffers from anxiety, that does not mean he/she has a disorder. Disorders are for the more severe form of anxiety - i.e. the child is unable to function at all in many situations. It might be wise to google the term "childhood anxiety" or "easing school jitters" or similar words/phrases to find more information about this issue. There should be information in some downloads, articles or books which should be able to guide you through this time.
By the way, the difference between being shy and displaying anxiety is function. When a child is unable to function, then we are dealing with anxiety. And, in some of the situations you described, your child appears to be suffering from anxiety. However, in some of the other situations, he appears to be able to function quite well (especially in the classroom with adult supervision). So, I really think with time and patience and educating yourself and the school staff on anxiety issues, things will slowly get better .
Is it possible some of the children he saw had not been too nice with him? children dont always want to open up on what is the problem , ask him specific questions about the School, his Teacher and the other Kids he is with as it does sound as if the problem lies there, the fact of withdrawal could be there is a problem with interaction with them.He could be plain old shy more than anxious.