The moment he asks you, not a second before, not a second after, you tell him, you tell him the whole truth, and he’ll be alright. Yeah it might hurt him, but in practice it will confront him to a problem, that will teach him a valuable leason: you can control how you feel about things.
Kids need to know this, emotional inteligence should be taught without compromises, kids are tough, they can take it.
Actually, its better if they take it while young, its a skill they need anyway in this competitive world, soft skills are a must, let me take it a little further, and predict that soft skills are the only ones that will matter as the future comes near, everything automatized, the science and technologies will be relegated to its easthetic and utilitary values respectively, human studies and social improvements through concious effort are much better goals for humanity than creating and endless energy machine or curing all diseases, that while truly important goals eventually we will reach them, and that will help us be at peace and happy with each other, yes, the goal of humanity must be complete realization of human beings in harmony with the universe, we must accept that our goal is to become one with the universe at a macro level, because if we are truly alone(and i don’t think we are) there is nobody to defeat out there, nothing to fight, no wars possible in the universe, all we have is our selves and the universe.
But coming back to our kid, yeah, teach him all that, he’ll be a good citizen of humanity.