I read something yesterday that says that Trump can't just drop him due to some rules around him already being made at the RNC. That said, he could surely back out on his own.
I am assuming that the money behind Project 2025 want him there so that he can be their applicator to the federal government, to make sure as much of their P2025 plans are implemented. Trump has a history of wanting the admin side of the Presidency implemented by others. I would think that JD is there for a reason and that Trump would have a lot of internal issues if he waivered on that.