Fat cat vs. Skinny kitten

[Catlover]

I now have a fairly portly looking Sylvester (I can't believe I was trying to get the calories into him just over a month ago!) and a rather bony 5 month old Dudley. Dudley eats like a horse and Sylvester seems to have upped his intake since he's arrived but how do I keep Sylvester slim while beefing out Dudley? I want to feed Dudley ad libitum but Sylvie scoffs it too. Any tips?
They've started wrestling each other by the way- Dudley holds his own but it's a case of heavy weight vs. super featherweight!!