You do not report his/her income on your return*. If it has to be reported, at all, it goes on his own return.
What you describe is unearned income. So, he will need to file a tax return if that income was more than $1150.
A case could be made that this is self employment (earned) income. Less of it would be subject to income tax**, but he would have to pay self employment (social security & medicare) tax on earnings over $400.
*If his only income is from interest and dividends, Alaska PFD or capital gains distributions shown on a 1099-DIV, there is a provision for entering it on your return, using form 8814.
**A dependents standard deduction is the greater of $1150 or his earned income + $400, but not more than $12,950.
