This is more a question of law, if historical law, and might be better on Law.SE. I am not a lawyer. (I'm not a historian either, but it's not a crime to practice history without a license).
What I can say about the relevant history of California is it was a bit unique. Unlike most of the western states, California skipped being an Organized Incorporated Territory. It went straight from being part of the Mexican Cession, an Unorganized Territory, to a state. Ok, what does that mean?
"Organized" means it has a government approved by an Organic Act of Congress. California Territory had a government, but Congress didn't get around to making it official. Officially it had a military governor. Traditionally the US Congress blesses a territory with an Enabling Act to go ahead and write up a state constitution, but they didn't for California because Congress was deadlocked over slavery. Californians went ahead and wrote up a state constitution anyway.
What about the US Constitution? Does it apply? Today we have the concept of "incorporated territory" to determine if a piece of territory is part of the United States or some quasi-national thing administered by Congress. But all that didn't exist in 1850, it came in as a result of island territories gained in the Spanish-American war 50 years later. I can infer that the US Constitution (probably) did not apply to the Mexican Cession so the people there were not automatically US citizens. Probably. Why?
The Compromise of 1850 which created the state of California goes out of its way to declare that the laws of the United States apply to what's left of the Mexican Cession (ie. they organize and incorporate them). Here's one example.
And be it further enacted, that the Constitution, and all laws of the United States which are not locally inapplicable, shall have the same force and effect within the said Territory of New Mexico as elsewhere within the United States.
31st Congress, Session I, Ch. 49, Sec. 17
However, it could have just been redundant. AFAIK this wasn't brought before the Supreme Court, but what if the Compromise of 1850 had failed and California been left in legal limbo for a few years longer? I believe this question would have gone to the Supreme Court. While I can't guess what SCOTUS would have done in 1850, I know what they did in 1922 in Balzac v. Porto Rico:
Had Congress intended to take the important step of changing the treaty status of Puerto Rico by incorporating it into the Union, it is reasonable to suppose that it would have done so by the plain declaration, and would not have left it to mere inference. Before the question became acute at the close of the Spanish War, the distinction between acquisition and incorporation was not regarded as important, or at least it was not fully understood and had not aroused great controversy. Before that, the purpose of Congress might well be a matter of mere inference from various legislative acts; but in these latter days, incorporation is not to be assumed without express declaration, or an implication so strong as to exclude any other view.
The situation in 1922 was different than 1850. In 1850 this was about a sparsely populated hunk of land contiguous with the United States and fully in line with Manifest Destiny that the US was eager to gobble up. The cases in 1922 involved inhabited, formerly Spanish islands that people then were not eager to see become full, voting citizens of the US. Would SCOTUS have ruled the same way in 1850 as they did in 1922? Did I mention I'm not a lawyer?
There's a great video by CGP Grey on the dizzying current situation.
And then there's the fact that California had just been part of Mexico and was full of Mexican citizens. This complicates things. It was dealt with in the Treaty of Guadalupe Hidalgo ending the Mexican-American War and defining the terms of who gets what land and what happens to the people there. Article VIII covers what happens to Mexican citizens who suddenly find themselves living in the United States. They can stay and still be Mexican citizens and retain all their property, but they have one year after the treaty is ratified to tell the US. Otherwise they become US citizens.
This was a special case specifically written into the treaty and approved by Congress. It doesn't sound like this guy was Mexican anyway. It's not even clear if he was living in California or just passing through on the day California became a state. If he didn't live there, that is he couldn't establish residency in California, I seriously doubt he'd be considered a citizen.
So what does the law making California a state say about this? Nothing that I can find. I think this is a reflection of a different time. The Federal government was much weaker (the Civil War is about to make it whole lot stronger) and so was national identity. You were a citizen of your state first, and your state was part of the US. It was the United States, not the United People. This, too, was about to change, but it points to a conclusion to this mess.
Let's assume he was a resident of California on the day it became a state. I say yes, he would be a citizen of the United States. This goes back to another question about citizenship, What was the original process for becoming a US citizen?. The answer is pretty long, but it boils down to that you're a citizen of your state/colony/territory first. If your state/colony/territory changes its allegiance and becomes part of the US, you also change your allegiance and become a citizen of the US. Now this was in the late 1780s, and it was just a Congressional ruling, they didn't even have citizenship laws yet, so it could have been overridden by law.
However if he was a citizen of some other country, perhaps a Germanic state, then he may not claim citizenship of California and may not be a citizen of the US. May.
Like I said, this is probably better asked on Law.SE.