If you can feed some of a softer cut of grass hay, that can increase protein levels. Though this will usually be a third cut, which can be a bit rich for some rabbits. So you may only be able to feed a small amount.
If this has been a change of pellet brand/type for him, that may be why he's not eating these new ones very well yet. He may just need some time to get used to the change and will gradually start eating more.
If you find you need to try a different pellet type, Sherwood pellets may be an option to try, or oxbow senior pellets.
There are some supplements that can be tried as well. If regular rolled oats don't cause any mushy cecotrope issues, a little bit of those can be fed. I fed a small amount of black oil sunflower seeds to my elderly rabbits. A little sweet potato or plain canned pumpkin can add calories, but limited as they're high in carbs.
Remember with all diet changes, it's best to introduce one thing at a time and only gradually increase if there are no signs of digestive upset. Grass hay is the exception to this as it's usually well tolerated when suddenly introduced, with the exception of richer cuts of hay like very leafy third cut hay.
And a caution about some higher protein hays like alfalfa and possibly third cut grass hay. These can also be higher in calcium, which can cause bladder and/or kidney issues for some rabbits, and so may not be an option.
Weight gain diet for underweight rabbits