What I do for my butterball Allen, which seems to be working, is put out the weight management for everyone. When Butterball Allen is not around, the kittens get a generous amount of "treats" (shhhh... don't tell them its regular cat food) which I dish out a few times a day. Its a bonding thing for us too... I hand feed them individual pieces. It takes half an hour or so, but I wouldn't trade treat time for the world. This way, the kittens get the additional nutrition and food they need, while Allen gets the weight control formula he needs.

PS: and added, unexpected bonus of the treat time is that they will come running when I shake the treat cup. Which has saved my sanity a few times when they hide. They come running EVERY time. Never fail. They love their treat time too