Regardless of the amendment you filed, he will not be able to efile. He will need to file by paper. No extension is needed.
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removing him as a dependent is all right on your return. it does not mean he can mark he can not be claimed as a dependent on his return. if you can claim him but don't he must still check that he can be claimed on his return.
you're are entitled to claim him if he is your qualifying child or relative and thus a dependent.
you can 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 he is not a full-time student, he's under 19 at the end of the tax year. If a full-time student, he's under 24 at the 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 filing joint to facilitate refund of taxes withheld or estimates paid
or you can claim him as a qualifying relative if all these tests are met
• his gross income for 2019 less than $4,200
• you provided over ½ his support
• he isn't a qualifying child of another taxpayer