any tax submission can be amended for three years from the date you filed, so certainly 2016-2018 can be amended. 2015 can be amended but go back and look at the date you filed as the window is closing on that one
before going through the work to amend, I would suggest validating that you can deduct your adult son by using the IRS app at this website:
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
you can check back in time a number of years just by answering the questions in the app
i don't know about NY
