Is he eating normally? I think I remember reading that he is in an e-collar. Some rabbits won't eat their normal amount with one of those on. Also, he won't be able to eat his cecals. If you can, try to offer his cecals to him (I know, yuck, but some rabbits will take them). If this doesn't work, supplement his B vitamins by giving him Nutrical. You could also call your vet and ask how much of a human's B vitamin complex pill you could give him. Crush it and sprinkle it on wet veggies.
What type of pellets are you feeding- timothy or alfalfa? He has a lot of bone growth to do, so if he is on timothy pellets you could give him a handful of alfalfa hay a day. Alfalfa also has more protein and energy.
What I have done for two of mine that didn't keep weight on well and had dietary limitations was I made a mix of sunflower seeds, flax seeds, and rolled oats. I gave them 1/4 teaspoon a day. They were dwarf hotots, so a larger breed could get more.
Hope that helps!
Edit: Just a thought, if he can't eat his cecals you may also want to keep Benebac on hand for an upset tummy, especially if changes to his diet upset his tummy. I usually keep some on hand anyway.