Did your son die of cancer? If so, it is not unusual for your body to dream out your grief pain as the same sort of cancer that took your son from you. So if he's shaking his head "No," it could indicate that your dream's message is that YOU are not dying, just because he did. I wonder if one of your present feeling-states is that you are dying of the disease that took him from you, of the cruel-fate that cancer can be. If that's true, then the dream shows him showing you that your fate cannot be the same as his, that you must rediscover a way to live without him.
Please respond with how you feel about my interpretation. Does his expression feel to you like I imagine it to be?
Being in the hospital with Cancer is a symbol of"I have a sickness unto death" -- like a Christian proto-existential theological book by Soeren Kierkegaard (1849). Your grief might be affecting a part of your psyche to expect to have this depressed mood until you die--esp since you can always find pundits who will say "You never get over the loss of a child." Having Cancer in the hospital symbolizes having a depression that cancer patients often end up wishing they could die to escape from--but being in the hospital means that adequate treatment for your SOUL (body symbolizes soul) is present to insure your recovery, whether it's speedy or slow and deliberate--since grieving is a soul process that naturally and necessarily TAKES TIME. (In fact you normally revisit your grief later over anybody really important to you on expected and unexpected occasions, but your connection to that grief can be like touching base with your ancestors, in devotion to the Great Chain of Being.) So your son's headshaking signifies that you won't need to bear this cancerous depression until you die, but you can instead let your living be a testimony to your love for him, and spread that love out to "all your children in this world, joys that we all share" (from a hymn I composed that's unpublished).