How do I enter a W2 for a dependent?

Yes, your son must file his own return.

Tax filing requirement (for dependents)

The minimum income requiring a dependent to file a federal tax return. 2020 filing requirements for dependents under 65: Earned income of at least $12,400, unearned income (like from investments or trusts) of at least $1,100, or self-employment income over $400.

Your dependent should file their own tax return. That way, any taxes withheld as shown in box 2 of the W-2 will be refunded to them if they earned less than $12,400 in 2020.

Do a separate return and be sure to check the box that he/she can be claimed as a dependent.  Since you indicate he/she is a dependent

You don't include their income on your return. It would only go on their return if they are required to file one.  They don't have to file if they only have W2 income under 12,400 but can file to get back any withholding taken out in boxes 2 or 17.  But you don't get boxes 4 or 6 back.

To file a separate return for a dependent in the Online version you need to set up a new account separate from the parents.  Online is only good for one return per account.  You can use the same email address for 5 accounts.  You can probably use the Free Edition or the Free File website

Https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online 

Be sure on the dependent return you check the box that says you can be claimed on someone else’s return.  If your only income is W2 and under 12,400 you do not have to file a return except to get back any withholding taken out.

Filing requirements for a dependent

https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-dependent/00/26111

More than likely your son can filee for free also. Check out https://freefile.intuit.com/