If he’s on pasture, why is he being fed? Typically while on pasture they don’t need hay, unless he’s being heavily worked, he may need the extra protein.
How is the fencing? How is he with this group of horses? If the fencing is safe and he gets along with the herd, just leave him out there. I’m sure he’ll be fine.
He might pace the fence because he’s used to coming in at night, but once he gets over that he’s not coming in, he’ll go back to grazing. You can also see if where you board now, would be willing to put him out for his pasture time at night, so you can see how he does.
They adjust fast and he’ll be happy to be out in pasture all the time to run and play at his whim.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."