If your son has already filed his tax return and it has been accepted by the IRS, the only way he will be able to change it is by amending it. I have included a Link below that will help him amend his 2016 tax return. When he amends, make sure he checks the box next to the sentence that reads "Someone else can claim me as a dependent on their tax return" and "Someone will claim me as a dependent in 2016".. That way, you will be able to claim him as a dependent on your 2016 tax return.
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
While your son is amending his return, you can be preparing your tax return claiming your son as your dependent. You will not be able to efile your tax return because the IRS has already received your son's tax return with his social security number, (SSN) and their computers automatically reject any subsequent tax returns using an SSN a second time. Your only option is to paper-file your 2016 tax return. To do this, you would select Paper-File instead of Efile when you are in the File section after completing your return. Then print, sign, date and mail your tax return by April 18, 2017.
