Worrying is a normal response for all children, but some children worry a lot more than others.
Here are two steps to take, if your child is one who worries a lot:
Identify what your child is worried about. Talk with your child about his concerns, but don't force it. Be patient and be calm. If you show that you're worried too, your child may become even more anxious. It may help if you create a relaxing environment by helping your child focus through deep breathing and thinking about good things, before moving on to subjects that cause him to worry.
Work with your child to come up with ways of dealing with the worry, such as role-playing and acting out the situation, or obtaining more information about the worrisome situation. For example, this website recommends many excellent children's books that deal with typical worries of young children. Reading them aloud together can be very reassuring to you and your child.
If your child's worries are really affecting how your child sleeps, eats and plays, or they really start to worry or frustrate you, consult your child's physician.