I have a question for you to consider: Is this anxiety effecting his life to the point that he does not function or that you avoid everyday experiences? If you say yes, or you have to think about it, then I would consider consulting a board certified child psychiatrist. It may seem that he is too young, but anxiety is a medical issue created by a cheimical response in his brain and if he cannot over come this by himself, then he might need help. Just like kids with too much stomach acid can't think away the reflux, kids with out of control neurotransmitter production can't control anxiety.
We had a child like this. Worried about everything until finally, she could not get on the bus to go to school. It was painful to watch her. Within 3 weeks of treating this issue, she was a different child. I kicked myself for waiting because the answer for our daughter was so simple. I would not hessitate if you think he is misserable, but you will likely get a lot of advice to not medicate, try diet, etc,etc.
Try this. Apply any advice you get to his kidneys instead of his brain. I mean, instead of getting anxiety, insert "pees too much." If you do, you will laugh and see that this, like any other medical problem, is not subject to what "we want" them to have (who wants their kid to have kidney deseas, but no one tries to come up with a different diagnosis to avoid it.) The brain is an organ in your child's body, like all the rest.
Hope this helps, there is real help and releif out there.
M.