I'd definitely just cut out grain all together. If he's an easy keeper he does not need the grain, so why give it to him?
As you said, moving him to another pasture or reducing the ammount of hay is not an option then there is nothing other than giving him more work that will take off the pounds.
When was he last wormed? Could be a worm belly also. Horses should be worked 6 weeks - 8 weeks, and switching worming medicines everytime.
But I'll bet ya almost anyhting he ain't as fat as my guy hehe