Tricky interpretation. If he's standing on the sideline and field the ball in the air, or on a bounce, sure. He's got one foot in, one out and picks the ball off the turf in bounds.
IMO that's illegal participation.
Here's the relevant section bolded to emphasize the particular situation:
It is illegal for a player of either team to go out of bounds, without contact with an opponent, and to reach back into or re-enter the field of play to touch or recover a loose ball.