You will not receive monthly payments for your stepson while you still have an amended 2020 return that has not yet been processed. You will be able to get the CTC on your 2021 tax return when you claim him as a dependent on your 2021 return.
Even during “normal” times it takes about four months for the IRS to process an amended return. During the pandemic and due to the severe backlog at the IRS it is taking much longer—six months or more for many amended returns. Do not expect quick results from amending. When the IRS issues a refund for an amended return it will be by check. They do not make direct deposits for refunds for amended returns. You can watch for information here:
https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1