He probably doesn't have to file that unless he got a 1099NEC for it.
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 Social Security or W2 income under 12,550 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. If they got a 1099NEC for it they do have to file a tax return for it.
To file a separate return for a dependent in the Online version you need to set up a new account separate from yours. 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
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 their only income is W2 and under 12,550 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-d...
