Let me be honest: Over the years I've probably asked myself thousands of times why God allows things like this to happen–and I still don't know the full answer. The Bible speaks of "the mystery of iniquity"–and that is what evil is: a mystery (2 Thessalonians 2:7, KJV).
But what I do know is this: No matter what happens to us, God still loves us and He still cares for us. He knows your pain and heartache, and He knows the grief you and your son's family are going through. Jesus' promise is for you: "Blessed are those who mourn, for they will be comforted" (Matthew 5:4). We may not always understand why evil things happen–but God does, and we can turn to Him for the comfort we need.
Take comfort in the fact that your son knew Christ and that now he is safely in heaven beyond the pain and sorrows of this world. And take comfort also in the fact that some day you will see him again.
No, in one sense your son was not healed–but in another sense he was. Now his cancer is gone, for in heaven there is "no more death or mourning or crying or pain, for the old order of things has passed away" (Revelation 21:4). Thank God for the hope we have in Christ!