This is a very tragic situation. You are correct that if your son were 21 he could petition for you.
If you are placed in removal proceedings, you can apply for cancellation of removal as a non-permanent resident before an Immigration Judge as long as you have been physically present in the United States for at least 10 years, have good moral character (generally no criminal history), and can establish that your removal from the U.S. would result in exceptional and extremely unusual hardship to your son or any other United States citizen or Lawful Permanent Resident immediate relative. This standard of proof is quite high, but depending on the gravity of your son's medical condition, your chances of success should increase. If the Immigration Judge finds that your son will suffer exceptional and extremely unusual hardship if you were to be removed then you should be granted lawful permanent resident status.
I hope this answers your question.