How do I file a tax return for deceased son with zero income to get 3rd economic stimulus payment from 2021?

My son passed away in January of 2021. His 3rd stimulus payment of $1400 arrived two months later in March. I could not deposit it, so I sent it back to the IRS to an address I found online and wrote “void” on the back of the check.
However, according to the IRS, I am eligible to claim that 3rd stimulus check since he was alive on January 1, 2021. Last year, I filed his 2020 taxes noting he was deceased. I received his 2nd stimulus check as a direct deposit for that year. But in 2021, he didn’t have any income since he passed in early January. How do I file a tax return for a deceased individual without adding in W2 info and no income without the form getting rejected or the TT software telling me I’m missing information?

Also, I remember there was a form I had to include to claim the stimulus rebate, but now I can’t find that form. Is it too late to get his rebate?

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