I think it all depends on the horse and the management at your stable. But if you want to be safe, talk to your vet about it. She could really tell you what the best action to take to cut weight on Sonny.
I do know that the amount of feed is supposed to go by body weight, and that all food should be weighed, not measured. What I mean is, instead of giving lets say a total of 8 flakes a day, they should be weighed so you know the weight of it.
I hope that's not too confusing. If you need clarification, just ask