Like Oreoboy said, if you aren't already seeing a psychiatrist, then I also suggest you try it out. Nightmares are often caused by negative emotions and events in real life, but some can't really be dealt with alone. It sounds like you're very worried about your son's health and that's why you're having nightmares about him.

By not paying attention to things in a dream, they will go away. If you're lucid and can't stop the thing from happening, close your eyes and hold your hands to your ears. Start singing a song and focus on it. You'll soon wake up or end up in a more pleasant scenario.

Why you're ending up in the same nightmare before is because you're afraid you will. Try to stay up for a while and get something else on your mind.