The point is, Ego created Quill to be an EXTENSION of him. It's in his name (Ego). The only reason he created Quill was to return and help him finally achieve his goal of becoming EVERYTHING by acting as an extra conduit of Ego's own power (cancer metaphor anyone?). That is what all of Ego's kids were made to become, but Quill was the only one that was a success (Ego killed all the others).
When Ego dies, his power dies with him, and thus Quill can no longer tap into it. He may biologically still be part celestial, but since Ego created him to be a conduit of his own powers, Ego being dead means those powers can no longer feed through Quill. This is why he does not use said powers in Infinity War or Endgame when they are most needed (like saving Gamora), because he can't.
As for him holding the Power Stone, that was back when Ego was still alive, and was even hinted to be the reason Ego found him. Ego could "feel" the power in Quill, because, again, Quill was an extension of him. Whether he can still do that now is up in the air, but his celestial powers sure didn't save him from being snapped.