You and hopefully your son can go thru these couple of pages to see if anything applies to you. There is also contact info where you can talk to someone about it or answer any questions for you.
Now, MEDICARE -
Since you have never work in this country, I don't believe you can get Medicare unless
1. You are married or widowed from a spouse who is/ was vested in the U.S.Medicare program.
2. Perhaps the form you have is talking about buying into the program, if you qualify for that - it is about $450 per month for Part A, then a monthly premium for Part B and Part D unless you could qualify for extra help after buying into Part A - that doesn't seem to be quite what you need based on your income.
3. Since the same government agency CMS (Center for Medicare and Medicaid Services) oversees both Medicare and Medicaid, maybe they are talking about re-certifying for Medicaid. I am pretty sure that is done at the state level - so is the form from the state government where you reside or Federal government? If there is a contact number, call and ask.
I am assuming that Medicaid is meeting your health needs because that seems to be what you qualify for based on the info given here. You are a legal immigrant, been here for at least 5-years and are low income - you are also an elderly person - that's for MEDICAID.
You might be able to buy a private health care insurance plan but that would be expensive.
A long shot would be to see if you, as your son's dependent, could be added to his health care policy - again, even if possible, expensive.
You might also contact your states Dept. of Aging to see if they have anything at the state level that could benefit you other than Medicaid. Some benefits might be based on the household income rather than just your own - food stamps, home heating subsidy.