In the past, I was filing for myself and spouse. Now my 17 year old son has income, but the dependent questions does not ask about his income

No, you can't prepare 3 returns at the same time. You can only prepare 2 returns together for spouses/common-law partners. You would need to start a new return for your son. To do this in TurboTax Online, select My Returns > Start A New Return from the top of the left side menu.