How do I include my sons income from a part time job he had??

You don't include their income on your return. It would only go on their return if they are required to file one.  To file a separate return for your dependent you need to set up a new account.  Online is only good for one return per account.  You can use the same email address for 5 accounts.

You can start another return in TurboTax Online by:

1. Logging out if you are in your account
2. Choose what TurboTax Online product you want to use for the second return
3. Create your account screen
4. Set up a new login for the second return
5. Start working on the new return

You can have up to 5 accounts can be tied to the same email address.

Be sure on their return they check the box that says they can be claimed on someone else’s return.  If their only income is W2 and under 14,600 they 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