you don't report him and you can do nothing to stop him from claiming the child on future returns. if you are the custodial parent, and assuming you otherwise qualify, you claim the child on your return. then at some point, the IRS will send each of you a request to amend your return. you don't and assuming he doesn't the IRS sends a second notice to each asking for proof (the notice will contain info on what constitutes proof - things like school records, medical bills, affidavits from neighbors, pictures of him and his room, etc). the IRS makes a decision on the proof submitted. the one who submits inadequate proof gets a bill for penalties, interest, and taxes. if you have not filed for 2018, 19 or 20 you do have the right to file those returns, if you filed them but did not claim the child you can file amended returns for those years claiming the child.
you are entitled to claim him if he is your qualifying child or relative and thus a dependent
you could claim him as a qualifying child if all these tests are met
- he has the same principal abode as you for more than ½ the tax year. Temporary absences like for school are ignored
- If not a full-time student, under 19 at the end of the tax year. If a full-time student, under 24 at end of the tax year.
- he hasn't provided over ½ his own support
- he didn't file a joint return unless there was no tax liability but merely filed jointly to facilitate refund of taxes withheld or estimates paid
or as he could be a qualifying relative if all these tests are met
- his gross income for 2020 is less than $4,300
- you provided over ½ his support
- he isn't a qualifying child of another taxpayer
