Ha, 40 acres is nothing. Try getting your horse when he's in 640 acres. That's a mile by a mile.
Anyway, I have my mare in a 20 acre pasture, and she comes when I call her name. It's kinda cool because when I go in the barn in the morning, she meets me in there because she knows I will give her a little treat.
As for my other horses who are out in the 640 acre pasture, I usually drive the 4-wheeler up to them and shake the bucket of grain and they come running. (I can also call my geldings name and he will come running).
But I would try getting him used to the whistle. Since he is in a small pasture right now, why don't you try standing a couple hundreds yards away and shake the feed bucket and blow the whistle at the same time. That way he would associate the whistle with a treat.