Many children come to my yoga class because they experience anxiety and are "worried". When there are many reasons why they might be worried, I find that books might help to verbalize what's going on inside their head.
I personally use "The huge bag of worries" with my own children. I like that it explains in a child's friendly way that "worries" might belong to someone else. Our children are like sponge and they feel how we feel. Therefore they might be anxious because of the grown-up problems and way of life.
With this book, you can also make some more concrete resource like make a real bag and place worries inside: ask your child to write or draw what he is worried about on a slip of paper and place each slip in the worry bag, help him to determine if the worry is his or his parents, or his school teacher - pressure to pass exam for example, his friends's issues... You may want to use one colour paper for each kind of worry (school, friends, parents, his/her own worry, world worry and so on....). That way your child will have a concrete vision of what are his real reasons to be anxious, those he can act upon (because there are his) and he might realize that he doesn't have so many in the end (when he has sorted away those that are not his).
Let me know if it helps your child!
I'm busy working on my blog posts. Watch this space!