The 14-year-old has already agreed a deal to join City when he turns 18 years of age.
Per The Athletic, City have an agreement with his club Philadelphia Union to make him the most expensive homegrown MLS transfer of all time.
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Sullivan is yet to sign a professional contract with the Union and will remain with the club for the next few years.
The goal is to ensure that he can develop in a more comfortable and less pressurized environment.
He will then make the switch to England when he is 18 to compete in the City Under-21 or first team.