To break the news, or not

My child hopes and believes in anything he is told. He even trusts that there is no relevant unknown, for if there was his parents must have already mentioned it. He is capable of a confusing naivety so burdening that forces me to choose between letting him keep the wisdom of the heart or training him a more informed judge of events...a suspecting mind!
How could I explain that some things exist only in as much as he believes but most are just the way they are? That among many other sad stories his friends from Spain may no longer feel the loss he still does, and his trip to visit them might be cancelled for that very reason at the core of tens of obstacles and excuses?
I am unable so far to find the way.