If Sugar is eating and drinking normally, pooping and peeing the same and behaving the same, Sugar may just be maintaining weight. They don't all grow the same.
At 8 months old, the bunny should be close to full grown... he may not gain any more weight. That may be the biggest he'll get.
As long as Sugar's behavior is the same and nothing else has changed, I wouldn't be too concerned. He may not need as much food as Daddy if he stays smaller.