After our daughter passed we had to deal with this question much younger, our girls were 3 when they asked.
We just told them that is the circle of life, we come into the world as babies and when it is our time we go to heaven whether we are old or still a young baby. Now as they have gotten older I have explained that most times this happens when we get old and our heart is tired or we can no longer fight a cold and it is time to rest and be healthy forever. But my girls ? why our baby died and I say that sometimes it happens to the young beacuse they are very sick or accidents can occur and they go early. But at a young age they desire to know where we go and I used heaven to say that when we go there we are no longer sick and we live a healthy happy new life. They then ask if we are alone and I say that all the people that we know will end up there so the ones there play and have fun with their friends that are already there.
It is such a hard question but I feel that honesty had made my children very understanding about life and death. They have a healthy knowledge of it. We try to give them enough info to answer their questions but not too much so they get confused. Just watch how he reacts to the info and if he presses for more give as much as you feel comfortable with. I feel it is better to prepare them than to leave them full of questions they might get answered by someone else in a way you don't approve.
Good luck.