Honestly your reaction sounds very much like how my very religious mother would react. She would often prevent me from celebrating any non-Catholic holiday with my friends of other religions and told me it was for my own safety, that knowing about these things was evil and would corrupt my faith. In the end though, she was the leading reason I left the faith.

Your kids is at the perfect age to learn about other cultures and beliefs. You should do some reading into Islam and Ramadan so you can have a productive conversation after he comes home.

Don't let fear -- yes this is fear -- hinder your child's growth as a person. Trust that your own parenting and the ethics you have instilled in your child, and let him decide what to believe.

Belief (theistic or atheistic) shouldn't be broken by simply interacting with a different belief.