I'm based on information on THIS link:
http://www.uscis.gov/policymanual/HTML/PolicyManual-Volume12-PartH-Chapter4.html
and as far as I understand, he automatically gained his citizenship through me.
I don't want to put him in any trouble, that the reason I put out the question. I also don't intend to have him claiming citizenship, if I was not 100% sure. But I do have a genuine question about what to check on the forms, as at this time, we have no proof of his citizenship, which we WILL have as soon as he receives his US passport, in a few weeks.
Thanks!