Since you can't monitor the amount of grass you're horse is ingesting I agree with Rachel it'd be better if he was not on grass. However if you have no other choice you can make your horse wear a muzzle about 15 hours everyday to prevent him from eating and thus lose weight.
But I highly recommend you to ask a vet how to proceed.
If your horse is on grass I'd take him off for now.
Lunging. Start out with 10 minutes of trotting and a little cantering. Then increase difficulty each time.