the first question is he your dependent or not. he's your qualifying child or qualifying relative - ie your dependent, if he's your dependent he must so indicate on his return regardless of whether you claim him or not. if he's your dependent you'll not be able to e-file and claim him. you may be able to e-file after his amended return is fully processed. the bad news is that the 1040-x for amended returns won't be available until about 3/25/21. then it could take the IRS 4-6 weeks to process it. this is the first year the IRS is allowing non-professional taxpayers to e-file amended returns so timings are up in the air. If you are entitled to claim him but don't you may lose out on significant credits and it may affect your filing status costing you even more in taxes. it is unknown if you will be able to e-file even after his return is amended. I, therefore, recommend that you mail in your return claiming him. what will likely happen is that your refund will take longer but you should get it based on your return as filed. At some point, if your stepson doesn't submit an amended return, the IRS will send each of you a request to amend your respective returns.
you can claim him/her as a qualifying child if all these tests are met
• he/she had the same principal abode as you for more than ½ the tax year. Temporary absences like for school are ignored
• if he/she is not a full-time student, he's/she’s under 19 at the end of the tax year. If a full-time student, he's/she’s under 24 at the end of the tax year.
• he/she hasn't provided over ½ his/her own support
• he/she didn't file a joint return unless there was no tax liability but merely filing jointly to facilitate refund of taxes withheld or estimates paid
or you can claim him/her as a qualifying relative if all these tests are met
• his/her gross income for 2020 was less than $4,300
• you provided over ½ his/her support
• he/she isn't a qualifying child of another taxpayer
