you say his w-2 was over $12,200 and that he did check he could be claimed as a dependent. the standard deduction would be the max $12,200 . I'm assuming he did not have a schedule C on his return which showed a loss or possibly a loss from a partnership showing negative self-employment income.
your son is not a qualifying child (you can't claim him as a dependent ) unless 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 jointly to facilitate refund of taxes withheld or estimates paid
also, you can not claim him as a qualifying relative (dependent) because it would seem his gross income is too high. the limit is $4200 for 2019 while his w-2 is much more.
my guess is your CPA is not returning calls because he gave you an incorrect answer. hopefully, the amended return for your son was not filed
