If your son sold stock at a profit then the boy must file his own return if it is required ... the stock sale IS income.
Please see the filing requirements for dependents:
- Your earned income (money you made by working) exceeds $12,400
- Your unearned income (interest, dividends, capital gains, etc.) exceeds $1,100
- Your business or self-employment net income (gross minus expenses) is at least $400
- Your gross income (earned plus unearned) exceeds the larger of $1,100 or your earned income (up to $12,050) plus $350
