Yeah, Karthik is right but that only applies before Omniverse, Ben wasn't afraid after that. Plus, Ghostfreaks true form is weak to sun light, meaning Ben wouldn't be able to use him during the day. So there would be no point to have him in his true form
He theoretically could use Ghostfreak's true form whenever he wants, given he's away from bright lights. Seeing as he's far more comfortable using Ghostfreak's shielded form now, he might eventually use the true form, but it's really up to him.