Someone who is single, under 65 years old, not blind, and can be claimed as a dependent is required to file a federal tax return for 2019 if any of the following apply.

1. His unearned income was over $1,100.
2. His earned income was over $12,200.
3. His gross income was more than the larger of (a) $1,100 or (b)
   his earned income (up to $11,850) plus $350.

He is also required to file a federal tax return if any of the following apply.

• He had net self-employment income of $400 or more.
• He received Advance Premium Tax Credit for health insurance from the Marketplace.
• He received distributions from an HSA, Archer MSA or Medicare Advantage MSA.
• He had wages of $108.28 or more from a church or qualified church-controlled organization that is exempt from
  employer social security and Medicare taxes.

I neglected to mention that the requirements I listed in my first reply above are for someone who is single, under 65 years old, and not blind. I have updated my original reply to reflect this.

In addition to the information provided by SuperUser rjs, if your son had federal/state tax withheld by his employer, he can file a tax return to seek a refund of federal and state tax withheld.  He just needs to say on his own return that he can be claimed as a dependent on someone else's return.

You can use the calculator at the following link on the IRS web site to determine whether someone is required to file a tax return. When the calculator says "you" or "your" it refers to your son. Answer the questions as if you were your son.

Do I Need to File a Tax Return?

Your state might have different filing requirements.