Did they deny the whole 4200 or just his 1400? Sounds like the IRS thinks they already sent it to you last year. You didn't get any letters 6475 saying how much they sent you? You should each get a letter for half (so 2100 each).
Check your IRS account (both accounts if married)
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
