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He can only make a contribution for the amount of money he made on his business. The maximum contribution is $6,000 in 2021.

Being self-employed, he has to also deduct one-half of self-employment tax from the amount he can contribute. For a $6,000 contribution on $6,000 of income, that amount is $$5576. If he contributed more, he over contributed.

Also, make sure he entered his 1099-NEC in the right place.