With only $3621 of earned income, he was not required to file a tax return. But with unearned income over $350 and gross income of more than $1050; he is now required to file a tax return.
If the $956 was qualified dividends ($956 was in both boxes 1a and box 1b of the 1099-DIV ), he will not owe any tax, but must still file an amended tax return.