Thank you! My son found out the same information in the FAQs at https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue. As of March 18, 2021, IRS is processing July 2019 returns. Another FAQ had the same answer that you gave. And entering 0 for AGI worked!
As for his 2019 return, he did not send it with tracking because the Post Office was crowded with pre-Christmas mailers. Fortunately, he doesn't owe any taxes so if his return was lost he can refile without paying interest.
