It depends, if the amount of the dividends for a dependent child is less than $1,100 then a return does not need to be filed to report the unearned income on the 1099-DIV. If the dividends are more than $1,100 but less than $11,000, then you can include your child's income on Form 8814 with your return. Enter at Less common income / Child's income. IRS Publication 929 discusses tax Rules for Children and Dependents.
But, it is usually best for him to file a separate return if the amount is more than $1,100, as qualified dividends and capital gain distributions could be taxed at a higher rate on the parent's return. (From @Hal_Al)
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