According to @Johnnybegood10 , Carl is with Schalke's u16s for the upcoming year. He's also in the team photo, which can be found here: http://knappenschmiede.schalke04.de...h/u16/mannschaftsfoto/page/181-3419-39--.html
His USSDA profile page, confirming US citizenship: http://rich.ussoccerda.com/sam/teams/index.php?team=1655741&player=68717397 PLAYER PROFILE Date of Birth: 01/11/2002 Position: Midfielder Hometown: Richmond , VA Nationality: United States Since he's only 15, we have to assume an EU passport (or a valid family move).
I think the local Schalke poster said he lives with his Germany-based grandparents in the Taitague thread?
Looked through all of Schalke's U16s matches so far, Carl has yet to make an appearance in any matchday squad. Maybe he is having some clearance issues or something like that? http://www.fussball.de/mannschaft/f...0VV0AG811VSOLMNRU#!/section/id-team-matchplan
He turns 16 on January 11, so if there's no family move and he does have a German/Austrian/Swiss passport (with a name like Hartmann, there's a decent shot) he should be able to make the move officially.
Here's an older story that gets into his German grandparents, his younger brother Peter (also a soccer player) and their travels in the game. https://www.fupa.net/berichte/sc-bayer-05-uerdingen-aus-den-usa-zum-real-madrid-camp-317797.html
It looks like he played a couple games for Richmond United's U-16/17 team in the USDA. http://rich.ussoccerda.com/sam/teams/index.php?team=7416176&player=536817591