For "dependent's income; shall we file our dependent, son, income separately or included in our tax as part of our income?

You have to file a separate tax return for your son. You will need to set up a different login for him since you can only complete one return per TurboTax Online account.

The dependent files their own return if they have a filing requirement or to get a refund of tax withheld.  You do not include the dependent ‘s income on your return.