Donald Trump Jr is in the same position as B Hussein Obama. No one is going to say he is not eligible to run for POTUS.
Democrats may elect ineligible candidates, but enough Republicans will not vote for one to make it pointless to nominate one.
Now ask yourself these questions...
Why would the U.S. Citizenship and Immigration Services, of all places, be asked to weigh in on this matter?
Do they have, say...jurisdiction over USC 8 issues?
The U.S. Citizenship and Immigration Services doesn't have jurisdiction over natural born citizens, do they?
The U.S. Citizenship and Immigration Services did say they were citizens and they didn't say they were natural born citizens...right?