A normally developing 2-year-old will test his or her limits. In addition, the child might not know how to express frustration, so pushing or hitting might be normal. However, children need to be taught that aggressive behaviors are not an acceptable way of dealing with frustration.
What do you think about your child's behaviors? Is he aggressive at home? If yes, are these new behaviors, or has that been going on for a while? Is there family history of aggression, or other mental health problems?
If his aggressive behaviors are new or have escalated over time, you might want to talk to his health care provider. If these behaviors happen only at the babysitter's, there must be something in that environment triggering these behaviors. Talk with the babysitter to get a better insight.