Says my son, and step kids aren't able to qualify as dependents because it asks if they're US citizens which they're not (my son isn't yet, but will be).
And as I said above, your son IS a US citizen. It's just that you don't have the official paperwork confirming it yet - so you will need to get his SSN before you can claim him on your taxes.
What makes him a US citizen is his blood and his DNA, not his passport or SSN (I have a SSN and I'm not a US citizen)... you are a US citizen, therefore he is a US citizen - it doesn't matter that he was born in the UK, he's still a US citizen - but you need to report his birth to the US Embassy so they have a record of him being a US citizen.
He won't be able to get an ITIN, because ITINs are only for people who are not eligible for a SSN. Your son is eligible for a SSN, it's just that you just haven't applied for it yet. His SSN will take about 6-8 weeks to arrive after the embassy appointment, and the ITIN applications can take 6 weeks to be processed.
If he was born in the US, his birth would already be reported to the government and you would already have his SSN - it's no different in the UK... he's still a US citizen, but you just haven't reported his birth to the US government or got his SSN yet.
So, when it asks you whether your kids are US citizens your answer should be that one is a US citizen (so you need to get his SSN first before you do the taxes) and two aren't US citizens (so if you want to claim them you need ITINs for them before you do the taxes).