There are a few things that you need to take into consideration when trying to find a way to reduce calories in a rabbits diet. Protein and carbs are the main dietary influences on weight gain, increased indigestible fiber will lead to weight loss. Since you really are feeding a minimal amount of pellets, dropping them may not make that much of a difference. So you have to look at the hay, forage, herbs, veg, that you feed.
The maturity of the hay when it is cut, will determine how high the nutrient value is, and essentially how fattening it will be. The leafier and greener the hay is, the higher in nutrients and protein it will be. The more mature a hay is and the more it contains stalky mature stems, the lower in protein it will be and higher in indigestible fiber, which also helps improve gastric motility. So with any grass hay, the leafier it is, the higher in protein and more fattening it is, the stalkier it is, the lower in protein it will be and thus less fattening.
Grain hays like oat, will tend to be a more fattening hay as you will either be feeding a less mature green leafy hay which will be higher in protein, or a more mature stalky hay that contains seed heads with grain that is high in carbs, and so will also be fattening. Unless you are wiling to pick the seed heads out of a mature oat hay, it is going to be too high in carbs to lead to any weight loss, and may actually cause weight gain. I personally would avoid feeding oat hay to a rabbit I am trying to encourage weight loss with.
Alfalfa is usually best avoided because not only is it very high in calcium, but is usually high in protein because it is generally cut at a time when it is very leafy and will thus be high in protein. It also just naturally contains a higher amount of protein than grass hays.
With forage, herbs, and veg, you will want to avoid anything that might be high in sugars, carbs, and starches.
Of course, it's not necessarily what you are feeding that is going to completely influence weight loss, but what your rabbit is actually consuming. If you are feeding a hay that is a mix of leaf and stems, and your rabbit won't eat the low protein stems but prefers to eat the high protein leafy bits, that will likely lead to weight gain. So you do have to look at what your rabbit is actually consuming and not just what is being offered.
You will also need to closely monitor your other rabbits weight. If he is at a good weight now, changing the diet to contain less protein, could lead to him losing weight, unless you were to continue to feed him his own special food separately.
Even with a concerted effort to change your buns diet, you may find that no significant weight loss occurs. I have a rabbit that has always been a little chunky, and no matter the diet changes I have tried, she just keeps the weight on her.