The Form 8814 can only be filed if your child's only income is unearned (interest, dividends, and capital gains). Because your child has W-2 income in addition to the unearned income, he will be required to file his own tax return reporting the W-2 income and the unearned income. If he qualifies as your dependent, you will still be able to claim him on your return, but when he files his return (or when you prepare it for him), he will need to mark that he can be claimed by someone else as a dependent. That area will be located in the Personal Info portion of the tax return in TurboTax Online. I will attach a screenshot of it to better help you/him locate it.
Please see the following TurboTax FAQ for information regarding filing requirements of a dependent:
https://ttlc.intuit.com/replies/3302185
