You are entitled to US citizenship if one parent is a US citizen that meets US residency requirements, and the residency requirements are much less stringent if both parents are US citizens, as in his case, to the extent that only one parent has to have been resident in the US for a nominal amount of time. So, the only way that he wouldn't be a US citizen would be if neither of his parents, although US citizens, had ever established residence in the US before his birth, which although possible is highly unlikely. He may never have made a claim to US citizenship, but neither have many people who were born here, if they have never left the country.

How do I know all this? We aren't US citizens, but our kids are. If we were to take them out of the US to live before they met the residency rules, then any grandkids that they might have later might not be US citizens. I suppose that is not entirely unreasonable, but it isn't much similar to other countries' rules, so under certain circumstances we could theoretically end up with grandkids who were stateless!

And as for social security, there is no requirement that you have to be a US citizen, although you have to at least be a legal resident, not an illegal or someone here on a visa. Otherwise I wouldn't qualify for it myself, but I do, or at least will at 65. The GOP at one time tried to pass a bill to deny people like me the social security we have paid for if we return to our home countries to retire, but mercifully it didn't pass.